Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	MESA
Entity File Number	001-38626
Entity Tax Identification Number	850302351
Entity Address, Address Line One	410 North 44th Street
Entity Address, Address Line Two	Suite 700
Entity Address, City or Town	Phoenix
Entity Address, Postal Zip Code	85008
City Area Code	602
Local Phone Number	685-4000
Entity Address, State or Province	Arizona
Entity Registrant Name	MESA AIR GROUP INC
Entity Central Index Key	0000810332
Current Fiscal Year End Date	--09-30
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		31,413,287

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Current assets:
Cash and cash equivalents	$ 79,909	$ 103,311
Marketable securities		19,921
Restricted cash	3,647	3,823
Receivables, net	7,902	14,290
Expendable parts and supplies, net	20,268	15,658
Prepaid expenses and other current assets	46,425	40,914
Total current assets	158,151	197,917
Property and equipment, net	1,286,022	1,250,829
Intangibles, net	9,984	11,341
Lease and equipment deposits	1,977	2,598
Other assets	9,849	9,703
Total assets	1,465,983	1,472,388
Current liabilities:
Current portion of long-term debt and capital leases	163,623	155,170
Accounts payable ($4,859 and $1,330 to related party)	33,585	54,307
Accrued compensation	14,492	12,208
Other accrued expenses	34,152	29,696
Total current liabilities	245,852	251,381
Noncurrent liabilities:
Long-term debt and capital leases, excluding current portion	717,546	760,177
Deferred credits ($6,288 and $7,702 to related party)	13,401	15,393
Deferred income taxes	50,695	39,797
Other noncurrent liabilities	25,755	31,173
Total noncurrent liabilities	807,397	846,540
Total liabilities	1,053,249	1,097,921
Commitments and contingencies (Note 13 and Note 14)
Stockholders' equity:
Preferred stock of no par value, 5,000,000 shares authorized; no shares issued and outstanding
Common stock of no par value and additional paid-in capital, 125,000,000 shares authorized; 30,361,168 (2019) and 23,902,903 (2018) shares issued and outstanding, 4,603,333 (2019) and 10,614,990 (2018) warrants issued and outstanding	237,613	234,683
Retained earnings	175,121	139,784
Total stockholders' equity	412,734	374,467
Total liabilities and stockholders' equity	$ 1,465,983	$ 1,472,388

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Statement Of Financial Position [Abstract]
Accounts payable to related party	$ 4,859	$ 1,330
Deferred credits to related party	$ 6,288	$ 7,702
Preferred stock, par value	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, no par value	$ 0	$ 0
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	30,361,168	23,902,903
Common stock, shares outstanding	30,361,168	23,902,903
Common stock, warrants issued	4,603,333	10,614,990
Common stock, warrants outstanding	4,603,333	10,614,990

Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating revenues:
Total operating revenues	$ 180,224	$ 171,739	$ 535,527	$ 504,063
Operating expenses:
Flight operations	53,025	51,795	155,636	155,602
Fuel	211	151	433	349
Maintenance	54,322	48,290	139,504	154,046
Aircraft rent	12,875	17,975	41,104	54,557
Aircraft and traffic servicing	978	848	2,977	2,592
General and administrative	12,435	22,066	38,121	43,333
Depreciation and amortization	19,761	16,013	57,528	47,611
Lease termination	9,540	15,109	9,540	15,109
Total operating expenses	163,147	172,247	444,843	473,199
Operating income	17,077	(508)	90,684	30,864
Other (expenses) income, net:
Interest expense	(13,496)	(14,118)	(42,110)	(41,592)
Interest income	733	11	1,188	30
Loss on extinguishment of debt			(3,616)
Other (expense) income, net	(451)	(15)	82	(117)
Total other (expense), net	(13,214)	(14,122)	(44,456)	(41,679)
Income (loss) before taxes	3,863	(14,630)	46,228	(10,815)
Income tax expense (benefit)	856	(3,495)	10,891	(24,676)
Net income (loss)	$ 3,007	$ (11,135)	$ 35,337	$ 13,861
Net income (loss) per share attributable to common shareholders
Basic	$ 0.09	$ (0.48)	$ 1.02	$ 0.59
Diluted	$ 0.09	$ (0.48)	$ 1.01	$ 0.58
Weighted-average common shares outstanding
Basic	34,835	23,336	34,683	23,298
Diluted	35,112	23,336	35,051	23,772
Contract Revenue [Member]
Operating revenues:
Total operating revenues	$ 170,366	$ 159,916	$ 510,586	$ 470,820
Pass Through and Other [Member]
Operating revenues:
Total operating revenues	$ 9,858	$ 11,823	$ 24,941	$ 33,243

Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Number of Warrants [Member]	Common Stock and Additional Paid-In Capital [Member]	Retained Earnings [Member]
Beginning balance at Sep. 30, 2017	$ 222,224			$ 114,456	$ 107,768
Beginning balance, shares at Sep. 30, 2017		11,294,083	12,230,625
Stock compensation expense	428			428
Cumulative effect of change in accounting principle	665				665
Net income	22,624				22,624
Ending balance at Dec. 31, 2017	245,941			114,884	131,057
Ending balance, shares at Dec. 31, 2017		11,294,083	12,230,625
Beginning balance at Sep. 30, 2017	222,224			114,456	107,768
Beginning balance, shares at Sep. 30, 2017		11,294,083	12,230,625
Repurchased shares and warrants		$ (500)
Net income	13,861
Ending balance at Jun. 30, 2018	232,623			112,732	119,891
Ending balance, shares at Jun. 30, 2018		12,572,630	10,871,730
Beginning balance at Dec. 31, 2017	245,941			114,884	131,057
Beginning balance, shares at Dec. 31, 2017		11,294,083	12,230,625
Stock compensation expense	391			391
Warrants converted to common stock, shares		1,089,720	(1,089,720)
Restricted shares issued, shares		10,412
Net income	2,372				2,372
Ending balance at Mar. 31, 2018	248,704			115,275	133,429
Ending balance, shares at Mar. 31, 2018		12,394,215	11,140,905
Stock compensation expense	390			390
Repurchased shares and warrants	(3,259)			(3,259)
Repurchased shares and warrants, shares		(74,275)	(250,000)
Warrants converted to common stock, shares		19,175	(19,175)
Restricted shares issued	326			326
Restricted shares issued, shares		233,515
Cumulative effect of change in accounting principle	(2,403)				(2,403)
Net income	(11,135)				(11,135)
Ending balance at Jun. 30, 2018	232,623			112,732	119,891
Ending balance, shares at Jun. 30, 2018		12,572,630	10,871,730
Beginning balance at Sep. 30, 2018	374,467			234,683	139,784
Beginning balance, shares at Sep. 30, 2018		23,902,903	10,614,990
Stock compensation expense	1,454			1,454
Stock issuance costs	157			157
Net income	19,081				19,081
Ending balance at Dec. 31, 2018	395,159			236,294	158,865
Ending balance, shares at Dec. 31, 2018		23,902,903	10,614,990
Beginning balance at Sep. 30, 2018	374,467			234,683	139,784
Beginning balance, shares at Sep. 30, 2018		23,902,903	10,614,990
Repurchased shares and warrants		$ (1,500)
Net income	35,337
Ending balance at Jun. 30, 2019	412,734			237,613	175,121
Ending balance, shares at Jun. 30, 2019		30,361,168	4,603,333
Beginning balance at Dec. 31, 2018	395,159			236,294	158,865
Beginning balance, shares at Dec. 31, 2018		23,902,903	10,614,990
Stock compensation expense	1,298			1,298
Repurchased shares and warrants	(449)			(449)
Repurchased shares and warrants, shares		(52,967)
Stock issuance costs	28			28
Warrants converted to common stock, shares		3,834,693	(3,834,693)
Restricted shares issued, shares		284,846
Net income	13,249				13,249
Ending balance at Mar. 31, 2019	409,285			237,171	172,114
Ending balance, shares at Mar. 31, 2019		27,969,475	6,780,297
Stock compensation expense	1,507			1,507
Repurchased shares and warrants	(1,065)			(1,065)
Repurchased shares and warrants, shares		(116,757)
Warrants converted to common stock, shares		2,176,964	(2,176,964)
Restricted shares issued, shares		331,486
Net income	3,007				3,007
Ending balance at Jun. 30, 2019	$ 412,734			$ 237,613	$ 175,121
Ending balance, shares at Jun. 30, 2019		30,361,168	4,603,333

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net income	$ 35,337	$ 13,861
Adjustments to reconcile net income to net cash flows provided by operating activities:
Depreciation and amortization	57,528	47,611
Stock compensation expense	4,259	1,209
Deferred income taxes	10,898	(22,139)
Amortization of unfavorable lease liabilities and deferred credits	(8,379)	(8,295)
Amortization of debt financing costs and accretion of interest on non-interest-bearing subordinated notes	3,147	3,577
Loss on extinguishment of debt	3,616
(Gain) loss on disposal of assets	(49)	276
Provision for obsolete expendable parts and supplies	477	74
Lease termination	9,540	15,109
Changes in assets and liabilities:
Receivables	5,903	(12,743)
Expendable parts and supplies	(5,076)	(677)
Prepaid expenses and other current assets	(7,861)	4,727
Accounts payable	(4,888)	8,361
Accrued liabilities	9,625	12,212
Net cash provided by operating activities	114,077	63,163
Cash flows from investing activities:
Capital expenditures	(116,731)	(98,509)
Sales of investment securities	20,077
Net returns of lease and equipment deposits	621	(2,673)
Net cash used in investing activities	(96,033)	(101,182)
Cash flows from financing activities:
Proceeds from long-term debt	163,658	187,703
Principal payments on long-term debt and capital leases	(197,409)	(158,170)
Debt financing costs	(4,870)	(5,767)
Debt prepayment costs	(1,672)
Stock issuance costs	185
Repurchase of stock	(1,514)	(540)
Net cash (used in) provided by financing activities	(41,622)	23,226
Net change in cash, cash equivalents and restricted cash	(23,578)	(14,793)
Cash, cash equivalents and restricted cash at beginning of period	107,134	60,347
Cash, cash equivalents and restricted cash at end of period	83,556	45,554
Supplemental cash flow information
Cash paid for interest	37,587	35,199
Cash paid for income taxes, net	415	368
Supplemental non-cash investing and financing activities
Accrued capital expenditures	$ 836	1,087
Acquisition of capital leases		$ 10,473

Organization and Operations

9 Months Ended

Jun. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Operations

1.	Organization and Operations

About Mesa Air Group, Inc.

Headquartered in Phoenix, Arizona, Mesa Air Group, Inc. ("Mesa" or the "Company") is a holding company whose principal subsidiary, Mesa Airlines, Inc. ("Mesa Airlines"), operates as a regional air carrier providing scheduled passenger service to 138 cities in 42 states, the District of Columbia, Canada, Mexico, Cuba, and the Bahamas. As of June 30, 2019, Mesa operated a fleet of 145 aircraft with approximately 761 daily departures and 3,400 employees. Mesa operates all of its flights as either American Eagle or United Express flights pursuant to the terms of the capacity purchase agreements entered into with American Airlines, Inc. (“American”) and United Airlines, Inc. (“United”).

The financial arrangements between the Company and its major airline partners involve a revenue-guarantee arrangement (i.e. a "capacity purchase agreement") whereby the major airline pays a monthly guaranteed amount for each aircraft under contract, a fixed fee for each block hour (the number of hours during which the aircraft is in revenue service, measured from the time of gate departure before take-off until the time of gate arrival at the destination) and flight flown and reimbursement of certain direct operating expenses in exchange for providing regional flying. The major airline partners also pay certain expenses directly to suppliers, such as fuel, ground operations and landing fees. Under the terms of these capacity purchase agreements, the major airline controls route selection, pricing and seat inventories, thereby reducing the Company's exposure to fluctuations in passenger traffic, fare levels, and fuel prices.

On August 8, 2018, the Company filed its Second Amended and Restated Articles of Incorporation, which, among other things: (i) effected a 2.5-for-1 stock split of its common stock; and (ii) increased the authorized number of shares of its common and preferred stock to 125,000,000 and 5,000,000, respectively. All references to share and per share amounts in the Company's condensed consolidated financial statements have been retrospectively revised to reflect the stock split and increase in authorized shares.

On August 14, 2018, the Company completed an initial public offering ("IPO") of its common stock, in which it issued and sold 9,630,000 shares (the "Firm Shares") of common stock at a public offering price of $12.00 per share, resulting in gross proceeds to the Company of approximately $115.6 million. Additionally, in connection with the IPO, the Company granted the underwriters an option to purchase up to an additional 1,444,500 shares of common stock at the same price. On September 11, 2018, the Company closed the sale of 1,344,500 shares ("Option Shares") of its common stock, in connection with the partial exercise of the overallotment option granted to the underwriters in its IPO. Of the 1,344,500 Option Shares sold, 723,985 were purchased directly from the Company and the remaining 620,515 shares were purchased directly from the selling shareholders. The Firm Shares and Option Shares were sold to the public for a price of $12.00 per share.

The sale of the Firm Shares and Option Shares raised gross proceeds of approximately $124.2 million. The Company did not receive any proceeds from the sale of the Option Shares by the selling shareholders. The Company received $111.7 million in net proceeds after deducting $8.7 million of underwriting discounts and commissions and $3.8 million in offering costs.

As part of the IPO, stock appreciation rights ("SARs") previously issued under the Mesa Air Group, Inc. Amended and Restated Stock Appreciation Rights Plan (the "SAR Plan"), which settled only in cash, were cancelled and exchanged for an aggregate of 1,266,034 shares of restricted common stock under the Company's 2018 Equity Incentive Plan (the "2018 Plan"), of which 966,022 were fully vested upon issuance and are included in the number of shares of common stock outstanding after the IPO. Of the 966,022 fully vested shares, 314,198 shares were retained by the Company to satisfy tax withholding obligations, resulting in a net issuance of 651,824 shares. Additionally, 983,113 shares of restricted common stock were issued to certain of its employees and directors under its 2018 Plan in exchange for the cancellation of 491,915 shares of existing unvested restricted phantom stock units and 491,198 shares of restricted stock under the 2011 and 2017 Plans, respectively.

American Capacity Purchase Agreement

As of June 30, 2019, the Company operated 62 CRJ-900 aircraft for American under a capacity purchase agreement (the "American Capacity Purchase Agreement"). Unless otherwise extended or amended, the capacity purchase agreement for the aircraft expires between 2021 and 2025. In exchange for providing flights and all other services under the agreement, the Company receives a fixed monthly minimum amount per aircraft, plus certain additional amounts based upon the number of flights and block hours flown during the month. In addition, the Company may also receive incentives or pay penalties based upon the Company's operational performance, including controllable on-time departure and controllable completion percentages. American also reimburses the Company for the actual amount incurred for certain items such as passenger liability and hull insurance, and aircraft property taxes. In addition, American also provides, at no cost to the Company, certain ground handling and customer service functions, as well as airport-related facilities and fuel. The Company also receives a monthly profit margin payment from American based on the number of aircraft operating. The capacity purchase agreement is subject to early termination for cause under specified circumstances and subject to the Company's right to cure under certain conditions. American had a 7.2% ownership interest in the Company, calculated on a fully-diluted basis as of June 30, 2019, and September 30, 2018. The related party amounts presented on the condensed consolidated balance sheets and statements of operations pertain to American.

On January 31, 2019, the Company entered into an amendment to the American Capacity Purchase Agreement, the terms of which provide for new and revised operational performance metrics, the Company’s right to earn additional incentive compensation based on the achievement of such metrics, and the right of American to permanently withdraw up to six (6) aircraft in the event the Company fails to meet such new/revised performance metrics. Under the terms of such amendment the Company agreed, effective April 2, 2019, to convert two (2) aircraft to be utilized by the Company as operational spares in the Company’s sole discretion throughout its system.

In July 2019, American exercised its right to permanently withdraw two (2) aircraft from the American Capacity Purchase Agreement due to the Company’s failure to meet certain performance metrics. The Company’s inability to use its operational spares during this measurement period contributed to the Company’s failure to meet such performance metrics.

United Capacity Purchase Agreement

As of June 30, 2019, the Company operated 20 CRJ-700 and 60 E-175 aircraft for United under a capacity purchase agreement (the "United Capacity Purchase Agreement"). Subject to certain early termination rights, the capacity purchase agreement for each of the 20 CRJ-700 aircraft expires between August and December 2019. Subject to early termination rights, the capacity purchase agreement for 30 of the E-175 aircraft (owned by United) expires between June 2019 and August 2020, subject to United's right to extend for four additional two-year terms (maximum of eight years). Subject to early termination rights, the capacity purchase agreement for 18 of the E-175 aircraft (owned by Mesa) expires between January 2028 and November 2028. The Company has an agreement with United to continue to operate the E-175 aircraft that began expiring in June 2019, pending the negotiation of a definitive agreement between the parties. While the Company does not have a similar arrangement with United regarding the CRJ-700 aircraft that begin expiring on August 31, 2019, the Company expects to continue to operate such aircraft as part of these overall discussions with United.

During fiscal 2017, the Company and United expanded the capacity purchase agreement to include, subject to early termination rights, an additional 12 E-175 aircraft (purchased by United) with the aircraft entering service through January 2018 for five-year terms, subject to United's right to extend for four additional two-year terms (maximum of eight years). In exchange for performing the flight services under such agreement, the Company receives from United a fixed monthly minimum amount per aircraft, plus certain additional amounts based upon the number of flights and block hours flown during the month. Additionally, certain costs incurred by the Company in performing the flight services are "pass-through" costs, whereby United agrees to reimburse the Company for the actual amounts incurred for the following items: property tax per aircraft, passenger liability insurance, and additionally for the E-175 aircraft owned by United, heavy airframe and engine maintenance, landing gear, auxiliary power units ("APU") and component maintenance. The Company also receives a profit margin based upon certain reimbursable costs under the agreement, as well as its operational performance in addition to a fixed profit margin. The capacity purchase agreement is also subject to early termination for cause under specified circumstances and subject to the Company's right to cure under certain circumstances. United is also permitted, subject to certain conditions, to terminate the agreement early in its discretion by giving Mesa notice of 90 days or more.

Summary of Significant Accounting Policies

9 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP") and include the accounts of the Company and its wholly owned operating subsidiaries. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification ("ASC") and Accounting Standards Update ("ASU") of the Financial Accounting Standards Board ("FASB"). All intercompany accounts and transactions have been eliminated in consolidation.

These condensed consolidated financial statements should be read in conjunction with, the Company's audited consolidated financial statements and notes thereto as of and for the year ended September 30, 2018 included in the Company's Annual Report on Form 10-K for the year ended September 30, 2018 on file with the U.S. Securities and Exchange Commission (the "SEC"). Information and footnote disclosures normally included in financial statements have been condensed or omitted in these condensed consolidated financial statements pursuant to the rules and regulations of the SEC and GAAP. These condensed consolidated financial statements reflect all adjustments that, in the opinion of management, are necessary to present fairly the results of operations for the interim periods presented.

The Company is an "emerging growth company," as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act,") and may remain an emerging growth company until the last day of its fiscal year following the fifth anniversary of the IPO, subject to specified conditions. The JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to "opt out" of such extended transition period, which means that when a standard is issued or revised and it has different application dates for public or private companies, the Company will be subject to the same new or revised accounting standards as other public companies that are not emerging growth companies.

Adoption of New Revenue Standard

On October 1, 2018, the Company adopted ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) ("ASU 2014-09" or "ASC 606") using the modified retrospective method. See Note 3: "Recent Accounting Pronouncements" for more information. To conform to ASC 606, the Company modified its revenue recognition policy as described below.

Revenue Recognition

The Company recognizes revenue when the service is provided under its capacity purchase agreements. Under these agreements, the major airline partners generally pay a fixed monthly minimum amount per aircraft, plus certain additional amounts based upon the number of flights and block hours flown. The contracts also include reimbursement of certain costs incurred by the Company in performing flight services. These costs, known as "pass-through costs," may include passenger and hull insurance as well as aircraft property taxes. Additionally, for the E-175 aircraft owned by United, the capacity purchase agreement provides that United will reimburse the Company for heavy airframe and engine maintenance, landing gear, APUs and component maintenance. The Company also receives compensation under its capacity purchase agreements for heavy maintenance expenses at a fixed hourly rate or per aircraft rate for all aircraft in scheduled service other than the E-175 aircraft owned by United. The contracts also include a profit margin on certain reimbursable costs, as well as a profit margin, incentives and penalties based on certain operational benchmarks. The Company is eligible to receive incentive compensation upon the achievement of certain performance criteria defined in the capacity purchase agreements. At the end of each period during the term of an agreement, the Company calculates the incentives achieved during that period and recognizes revenue attributable to the agreement during the period accordingly, subject to the variable constraint guidance under ASC 606. All revenue recognized under these contracts is presented as the gross amount billed to the major airline partners.

Under the capacity purchase agreements, the Company has committed to perform various activities that can be generally classified into in-flight services and maintenance services. When evaluating these services, the Company determined that the nature of its promise is to provide a single integrated service, flight services, because its contracts require integration and assumption of risk associated with both services to effectively deliver and provide the flights as scheduled over the contract term. Therefore, the in-flight services and maintenance services are inputs to that combined integrated flight service. Both the services occur over the term of the agreement and the performance of maintenance services significantly effects the utility of the in-flight services. The Company's individual flights flown under the capacity purchase agreements are deemed to be distinct and the flight service promised in the capacity purchase agreements represents a series of services that should be accounted for as a single performance obligation. This single performance obligation is satisfied over time as the flights are completed. Therefore, revenue is recognized when each flight is completed.

In allocating the transaction price, variable payments (i.e. billings based on flights and block hours flown, pass-through costs, etc.) that relate specifically to the Company's efforts in performing flight services are recognized in the period in which the individual flight is completed. The Company has concluded that allocating the variability directly to the individual flights results in an overall allocation meeting the objectives in ASC 606. This results in a pattern of revenue recognition that follows the variable amounts billed from the Company to their customers.

A portion of the Company's compensation under its capacity purchase agreements with American and United is designed to reimburse the Company for certain aircraft ownership costs. The Company has concluded that a component of its revenue under these agreements is deemed to be lease revenue, as such agreements identify the "right of use" of a specific type and number of aircraft over a stated period-of-time. The lease revenue associated with the Company's capacity purchase agreements is accounted for as an operating lease and is reflected as contract revenue on the Company's condensed consolidated statements of operations. The Company recognized $54.5 million and $53.5 million of lease revenue for the three months ended June 30, 2019 and 2018, respectively, and $164.5 million and $162.0 million of lease revenue for the nine months ended June 30, 2019 and 2018, respectively. The Company has not separately stated aircraft rental income and aircraft rental expense in the condensed consolidated statements of operations because the use of the aircraft is not a separate activity of the total service provided.

The Company's capacity purchase agreements are renewable periodically and contain provisions pursuant to which the parties could terminate their respective agreements, subject to certain conditions as described in Note 1. The capacity purchase agreements also contain terms with respect to covered aircraft, services provided and compensation as described in Note 1. The capacity purchase agreements are amended from time to time to change, add or delete terms of the agreements.

The Company's revenues could be impacted by a number of factors, including amendment or termination of its capacity purchase agreements, contract modifications resulting from contract renegotiations, its ability to earn incentive payments contemplated under applicable agreements, and settlement of reimbursement disputes with the Company's major airline partners. In the event contracted rates are not finalized at a quarterly or annual financial statement date, the Company evaluates the enforceability of its contractual terms and when it has an enforceable right, it estimates the amount the Company expects to be entitled to that is subject to the ASC 606 constraint.

The Company's capacity purchase agreements contain an option that allows its major airline partners to assume the contractual responsibility for procuring and providing the fuel necessary to operate the flights that it operates for them. Both of the Company's major airline partners have exercised this option. Accordingly, the Company does not record an expense or revenue for fuel and related fueling costs for flying under its capacity purchase agreements. In addition, the Company's major airline partners also provide, at no cost to the Company, certain ground handling and customer service functions, as well as airport-related facilities and gates at their hubs and other cities. Services and facilities provided by the Company's major airline partners at no cost are presented net in its condensed consolidated financial statements; hence, no amounts are recorded for revenue or expense for these items.

Contract Liabilities

Contract liabilities consist of deferred credits for cost reimbursements from major airline partners related to aircraft modifications associated with capacity purchase agreements and pilot training. The deferred credits are recognized over time depicting the pattern of transfer of control of services resulting in ratable recognition of revenue over the remaining term of the capacity purchase agreements.

Current and non-current deferred credits are recorded to other accrued expenses and non-current deferred credits in the condensed consolidated balance sheets. The Company's total current and non-current deferred credit balances at June 30, 2019 and September 30, 2018 are $13.4 million and $15.4 million, respectively. The Company recognized $1.3 million and $1.0 million of the deferred credits to revenue in the condensed consolidated statements of operations during the three months ended June 30, 2019 and 2018, respectively, and $3.8 million and $3.2 million during the nine months ended June 30, 2019 and 2018, respectively.

Contract Assets

The Company recognizes assets from the costs incurred to fulfill a contract including aircraft painting and reconfiguration and flight service personnel training costs. These costs are amortized based on the pattern of transfer of the services in relation to flight hours over the term of the contract. Contract assets are recorded as other assets in the condensed consolidated balance sheets. The Company's contract assets balances at June 30, 2019 and September 30, 2018 are $4.5 million and $4.6 million, respectively. Contract cost amortization was $0.6 million and $0.5 million for the three months ended June 30, 2019 and 2018, respectively, and $1.8 million and $1.4 million for the nine months ended June 30, 2019 and 2018, respectively.

Use of Estimates

The preparation of the Company's condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues, and expenses and the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements. Actual results could differ from those estimates.

Maintenance Expense

The Company operates under a Federal Aviation Administration ("FAA") approved continuous inspection and maintenance program. The Company uses the direct expense method of accounting for its maintenance of regional jet engine overhauls, airframe, landing gear, and normal recurring maintenance wherein the Company recognizes the expense when the maintenance work is completed, or over the repair period, if materially different. For leased aircraft, the Company is subject to lease return provisions that require a minimum portion of the "life" of an overhaul be remaining on the engine at the lease return date. The Company estimates the cost of maintenance lease return obligations and accrues such costs over the remaining lease term when the expense is probable and can be reasonably estimated.

Under the Company's aircraft operating lease agreements and FAA operating regulations, it is obligated to perform all required maintenance activities on its fleet, including component repairs, scheduled air frame checks and major engine restoration events. The Company estimates the timing of the next major maintenance event based on assumptions including estimated usage, FAA-mandated maintenance intervals and average removal times as recommended by the manufacturer. The timing and the cost of maintenance are based on estimates, which can be impacted by changes in utilization of its aircraft, changes in government regulations and suggested manufacturer maintenance intervals. Major maintenance events consist of overhauls to major components.

Engine overhaul expense totaled $11.4 million and $13.1 million for the three months ended June 30, 2019, and 2018, respectively, of which $1.9 million and $4.6 million, respectively, was pass-through expense. Engine overhaul expense totaled $20.7 million and $46.2 million for the nine months ended June 30, 2019, and 2018, respectively, of which $3.0 million and $9.8 million, respectively, was pass-through expense. Airframe C-check expense totaled $4.4 million and $6.3 million for the three months ended June 30, 2019, and 2018, respectively, of which $0 million and $1.6 million, respectively, was pass-through expense. Airframe C-check expense totaled $10.6 million and $19.8 million for the nine months ended June 30, 2019, and 2018, respectively, of which $0 million and $7.3 million, respectively, was pass-through expense.

Change in Accounting Policy

Stock Appreciation Rights (“SARs”) and Phantom Stock historically were accounted for as liability compensatory awards under ASC 710, Compensation – General, valued using the intrinsic value method, as permitted by ASC 718, Compensation – Stock Compensation, for nonpublic entities. Upon becoming a public company, as defined in ASC 718, in the third quarter of fiscal 2018, the Company was required to change its methodology for valuing the SARs and Phantom Stock. The SARs and Phantom Stock were re-measured at each quarterly reporting date and were accounted for prospectively at fair value using a Black-Scholes fair value pricing model until they were converted to restricted stock awards upon completion of the Company’s IPO. The Company recorded the impact of the change in valuation methods as a cumulative effect of a change in accounting principle, as permitted by ASC 250, Accounting Changes and Error Corrections. The effect of the change increased the SARs and Phantom Stock liability by $2.4 million which was the difference in compensation cost measured using the intrinsic value method and the fair value method. An equal and offsetting change to retained earnings in the consolidated balance sheet was recorded with the revaluation. Any future changes in fair value were recorded as compensation expense in the consolidated statement of operations. Upon completion of the Company’s IPO the SARs and Phantom Stock were cancelled and exchanged for shares of restricted stock under our 2018 Plan.

Recent Accounting Pronouncements

9 Months Ended

Jun. 30, 2019

Accounting Changes And Error Corrections [Abstract]

Recent Accounting Pronouncements

3.	Recent Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606). ASC 606 establishes a new recognition model that requires revenue to be recognized in a manner to depict the transfer of goods or services to a customer at an amount that reflects the consideration expected to be entitled in exchange for those goods or services. On October 1, 2018, the Company adopted this ASU using the modified retrospective method. Under the new standard, the Company concluded that, in addition to the aircraft lease, the individual flights are distinct services and the flight services promised in the capacity purchase agreements represent a series of services that should be accounted for as a single performance obligation. Revenue is recognized over time as the flights are completed. The adoption of this ASU did not have an impact on recorded amounts when applied to the opening balance sheet as of October 1, 2018. The adoption did not impact the condensed consolidated financial statements presented other than the disclosures noted in Note 2.

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments—Overall (Subtopic 825-10), Recognition and Measurement of Financial Assets and Financial Liabilities ("ASU 2016-01"), which provides guidance related to the classification and measurement of financial instruments. The guidance primarily impacts the accounting for equity investments other than those accounted for using the equity method of accounting, financial liabilities under the fair value option and the presentation and disclosure requirements for financial instruments. The accounting for other financial instruments, such as loans, investments in debt securities and financial liabilities is largely unchanged. The guidance is effective for fiscal years beginning after December 15, 2017 with early adoption permitted. The Company has adopted ASU 2016-01 effective October 1, 2018; the adoption of this standard did not have a material impact on the financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which was subsequently amended and clarified and provides guidance requiring lessees to recognize a right-of-use asset and a lease liability on the balance sheet for substantially all leases, with the exception of short-term leases. Leases will be classified as either financing or operating, with classification affecting the pattern of expense recognition in the statement of income. The guidance is effective for annual periods beginning after December 15, 2018, with early adoption permitted. The Company is currently evaluating the potential impact of adopting this new guidance on its condensed consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the FASB Emerging Issues Task Force), which clarifies how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The new standard is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. The Company adopted the standard effective October 1, 2018; the adoption of this standard did not have a material impact on the financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force), that requires restricted cash and cash equivalents to be included with cash and cash equivalents on the statement of cash flows. The new standard is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. The Company adopted the standard effective October 1, 2018 and modified the presentation to include changes in restricted cash in the Company's Condensed Consolidated Statement of Cash Flows.

Concentrations

9 Months Ended

Jun. 30, 2019

Risks And Uncertainties [Abstract]

Concentrations

4.	Concentrations

At June 30, 2019, the Company had capacity purchase agreements with American and United. All of the Company's condensed consolidated revenue for the nine months ended June 30, 2019 and 2018 and accounts receivable at June 30, 2019 and September 30, 2018 was derived from these agreements. The terms of both the American and United capacity purchase agreements are not aligned with the lease obligations on the aircraft performing services under such agreements.

Amounts billed by the Company under capacity purchase agreements are subject to the Company's interpretation of the applicable capacity purchase agreement and are subject to audit by the Company's major airline partners. Periodically, the Company's major airline partners dispute amounts billed and pay amounts less than the amount billed. Ultimate collection of the remaining amounts not only depends upon the Company prevailing under the applicable audit, but also upon the financial well-being of the major airline partner. As such, the Company periodically reviews amounts past due and records a reserve for amounts estimated to be uncollectible. The allowance for doubtful accounts was $1.2 million and $1.3 million at June 30, 2019 and September 30, 2018, respectively. If the Company's ability to collect these receivables and the financial viability of its partners is materially different than estimated, the Company's estimate of the allowance could be materially impacted.

American accounted for approximately 53% and 54% of the Company's total revenue for the three months ended June 30, 2019 and 2018, respectively, and 54% and 57% for the nine months ended June 30, 2019 and 2018, respectively. United accounted for approximately 47% and 46% of the Company's revenue for the three months ended June 30, 2019 and 2018, respectively, and 46% and 43% for the nine months ended June 30, 2019 and 2018, respectively. A termination of either the American or the United capacity purchase agreement would have a material adverse effect on the Company's business prospects, financial condition, results of operations, and cash flows.

Intangible Assets

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Intangible Assets

5.	Intangible Assets

Information about the intangible assets of the Company at June 30, 2019 and September 30, 2018, were as follows (in thousands):

	June 30,			September 30,
	2019			2018
Customer relationship	$	43,800		$	43,800
Accumulated amortization		(33,816	)		(32,459	)
	$	9,984		$	11,341

Total amortization expense recognized was approximately $0.5 million and $0.1 million for the three months ended June 30, 2019 and 2018, respectively, and $1.4 million and $0.3 million for the nine months ended June 30, 2019 and 2018, respectively. The Company expects to record amortization expense of $0.5 million for the remainder of 2019, and $1.5 million, $1.2 million, $1.0 million, $0.9 million for fiscal years 2020, 2021, 2022, and 2023 respectively.

Balance Sheet Information

9 Months Ended

Jun. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Information

6.	Balance Sheet Information

Certain significant amounts included in the Company's condensed consolidated balance sheet as of June 30, 2019 and September 30, 2018, consisted of the following (in thousands):

	June 30,			September 30,
	2019			2018
Expendable parts and supplies, net:
Expendable parts and supplies	$	23,957		$	18,907
Less: obsolescence and other		(3,689	)		(3,249	)
	$	20,268		$	15,658

Prepaid expenses and other current assets:
Prepaid aircraft rent	$	39,785		$	30,267
Unutilized manufacturer credits		1,500			4,500
Deferred offering and reimbursed costs		2,090			1,945
Other		3,050			4,202
	$	46,425		$	40,914
Property and equipment, net:
Aircraft and other flight equipment substantially pledged	$	1,577,052		$	1,502,940
Other equipment		4,757			3,721
Leasehold improvements		2,797			2,754
Vehicles		924			692
Building		699			699
Furniture and fixtures		291			287
Total property and equipment		1,586,520			1,511,093
Less: accumulated depreciation		(300,498	)		(260,264	)
	$	1,286,022		$	1,250,829

Other accrued expenses:
Accrued property taxes	$	7,667		$	6,981
Accrued interest		7,729			6,118
Accrued vacation		5,894			5,470
Accrued wheels, brakes and tires		1,567			1,452
Other		11,295			9,675
	$	34,152		$	29,696

Depreciation expense totaled approximately $19.3 million and $15.9 million for the three months ended June 30, 2019 and 2018, respectively, and $56.2 million and $47.3 million for the nine months ended June 30, 2019 and 2018, respectively.

The Company recorded amortization of the unfavorable lease liability for approximately $1.4 million and $1.7 million for the three months ended June 30, 2019 and 2018, respectively, and $4.5 million and $5.1 million for the nine months ended June 30, 2019 and 2018, respectively, as a reduction to lease expense.

Fair Value Measurements

9 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

7.	Fair Value Measurements

The carrying values reported in the condensed consolidated balance sheets for cash and cash equivalents, accounts receivable, and accounts payable approximate fair value because of the immediate or short-term maturity of these financial instruments. Marketable securities are reported at fair value based on market quoted prices in the condensed consolidated balance sheets.

The Company's debt agreements are not traded on an active market. The Company has determined the estimated fair value of its debt to be Level 3, as certain inputs used to determine the fair value of these agreements are unobservable and, therefore, could be sensitive to changes in inputs. The Company utilizes the discounted cash flow method to estimate the fair value of Level 3 debt.

The carrying value and estimated fair value of the Company's long-term debt, including current maturities, were as follows (in thousands):

	June 30, 2019				September 30, 2018
	Carrying		Fair		Carrying		Fair
	Value		Value		Value		Value
Long-term debt, including current maturities⁽¹⁾	$	896,730	$	917,265	$	930,207	$	926,229

⁽¹⁾	Current and prior period long-term debts' carrying and fair values exclude net debt issuance costs.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Long-Term Debt and Other Borrowings

9 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Long-Term Debt and Other Borrowings

8.	Long-Term Debt and Other Borrowings

Long-term debt as of June 30, 2019 and September 30, 2018, consisted of the following (in thousands):

	June 30,			September 30,
	2019			2018
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2019⁽¹⁾⁽²⁾	$	—		$	4,428
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2022⁽³⁾⁽⁴⁾		54,513			69,340
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2024⁽⁵⁾		64,090			72,438
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2027⁽⁶⁾		113,861			122,591
Notes payable to secured parties, collateralized by the underlying
aircraft, due 2028⁽⁷⁾		200,322			209,240
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2028⁽⁸⁾		156,573			167,269
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2022⁽¹⁷⁾		77,910			95,060
Notes payable to financial institution, collateralized by the underlying
equipment, due 2022⁽⁹⁾	—				88,162
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2022⁽¹⁰⁾		51,421			63,403
Notes payable to financial institution, collateralized by the underlying
equipment, due 2020⁽¹¹⁾		2,073			3,318
Notes payable to financial institution due 2020⁽¹²⁾		2,837			4,360
Notes payable to financial institution, collateralized by the underlying
equipment, due 2020⁽¹³⁾		9,753			14,971
Notes payable to financial institution due 2019⁽¹⁴⁾		—			5,896
Working capital draw loan, collateralized by certain flight equipment
and spare parts⁽¹⁵⁾		—			—
Other obligations due to financial institution, collateralized by the underlying
equipment, due 2023⁽¹⁶⁾		8,840			9,731
Notes payable to financial institution, collateralized by the underlying
equipment, due 2024⁽¹⁸⁾		84,537			—
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2023⁽¹⁹⁾		70,000			—
Total long-term debt		896,730			930,207
Less current portion		(163,623	)		(155,170	)
Less unamortized debt issuance costs		(15,561	)		(14,860	)
Long-term debt—excluding current portion	$	717,546		$	760,177

⁽¹⁾	In fiscal 2005, the Company financed five CRJ-900 aircraft with $118.0 million in debt. The debt bears interest at the monthly London Inter-bank Offered Rate ("LIBOR"), plus 3.00% and requires monthly principal and interest payments.

⁽²⁾	In fiscal 2004, the Company financed five CRJ-700 and nine CRJ-900 aircraft with $254.7 million in debt. The debt bears interest at the monthly LIBOR plus 3.00% and requires monthly principal and interest payments.

⁽³⁾	In fiscal 2007, the Company financed three CRJ-900 and three CRJ-700 aircraft for $120.3 million. The debt bears interest at the monthly LIBOR plus 2.25% (4.638% at June 30, 2019) and requires monthly principal and interest payments.

⁽⁴⁾	In fiscal 2014, the Company financed 10 CRJ-900 aircraft for $88.4 million. The debt bears interest at the monthly LIBOR plus a spread ranging from 1.95% to 7.25% (4.338% to 9.638% at June 30, 2019) and requires monthly principal and interest payments.

⁽⁵⁾	In fiscal 2014, the Company financed eight CRJ-900 aircraft with $114.5 million in debt. The debt bears interest at 5.00% and requires monthly principal and interest payments.

⁽⁶⁾

In fiscal 2015, the Company financed seven CRJ-900 aircraft with $170.2 million in debt. The senior notes payable of $151.0 million bear interest at monthly LIBOR plus 2.71% (5.098% at June 30, 2019) and require monthly principal and interest payments. The subordinated notes payable are noninterest-bearing and become payable in full on the last day of the term of the notes. The Company has imputed an interest rate of 6.25% on the subordinated notes payable and recorded a related discount of $8.1 million, which is being accreted to interest expense over the term of the notes.

⁽⁷⁾	In fiscal 2016, the Company financed 10 E-175 aircraft with $246.0 million in debt under an EETC financing arrangement (see discussion below). The debt bears interest ranging from 4.75% to 6.25% and requires semi-annual principal and interest payments.

⁽⁸⁾

In fiscal 2016, the Company financed eight E-175 aircraft with $195.3 million in debt. The senior notes payable of $172.0 million bear interest at the three-month LIBOR plus a spread ranging from 2.20% to 2.32% (4.500% to 4.652% at June 30, 2019) and require quarterly principal and interest payments. The subordinated notes payable bear interest at 4.50% and require quarterly principal and interest payments.

⁽⁹⁾	In fiscal 2017, the Company financed certain flight equipment with $99.1 million in debt. The debt bears interest at the monthly LIBOR (rounded to the nearest 16th) plus 7.25% (9.638% at June 30, 2019) and requires monthly principal and interest payments.

⁽¹⁰⁾

In December 2017, the Company refinanced nine CRJ-900 aircraft with $74.9 million in debt. The senior notes payable of $46.9 million bear interest at the three-month LIBOR plus 3.50% (5.832% at June 30, 2019) and require quarterly principal and interest payments. The subordinated notes payable bear interest at the three-month LIBOR plus 4.50% (6.832% at June 30, 2019) and require quarterly principal and interest payments.

⁽¹¹⁾	In fiscal 2015, the Company financed certain flight equipment with $8.3 million in debt. The debt bears interest at 5.163% and requires monthly principal and interest payments.

⁽¹²⁾	In fiscal 2015 and 2016, the Company financed certain flight equipment maintenance costs with $10.2 million in debt. The debt bears interest at the monthly LIBOR plus 3.07% (5.402% at June 30, 2019) and requires quarterly principal and interest payments.

⁽¹³⁾

In fiscal 2016 and 2017, the Company financed certain flight equipment maintenance costs with $11.9 million in debt. The debt bears interest at the three-month LIBOR plus a spread ranging from 2.93% to 2.96% (5.262% to 5.292% at June 30, 2019) and requires quarterly principal and interest payments. The debt is subject to a fixed charge ratio covenant. As of June 30, 2019, the Company was in compliance with this covenant.

⁽¹⁴⁾

In fiscal 2017, the Company financed certain flight equipment maintenance costs with $25 million in debt. The debt bears interest at the three-month LIBOR plus 3.30% (5.632% at June 30, 2019) and requires quarterly principal and interest payments. The debt is subject to a fixed charge ratio covenant. As of June 30, 2019, the Company was in compliance with this covenant.

⁽¹⁵⁾

In fiscal 2016, the Company obtained a $35.0 million working capital draw loan, which terminates in August 2019. Interest is assessed on drawn amounts at one-month LIBOR plus 4.25% (6.638% at June 30, 2019). As of June 30, 2019, there were no borrowings outstanding under this facility. The working capital draw loan is subject to an interest and rental coverage ratio covenant. As of June 30, 2019, the Company was in compliance with this covenant.

⁽¹⁶⁾	In February 2018, the Company leased two spare engines. The leases were determined to be capital as the leases contain a bargain purchase option at the end of the term. Imputed interest is 9.128% and the leases requires monthly payments.

⁽¹⁷⁾

In June 2018, the Company refinanced six CRJ-900 aircraft with $27.5 million in debt and financed nine CRJ-900 aircraft, which were previously leased, with $69.6 million in debt. The senior notes payable of $65.8 million bear interest at the three-month LIBOR plus 3.50% (5.832 % at June 30, 2019) and require quarterly principal and interest payments. The subordinated notes payable of $29.8 million bear interest at three month LIBOR plus 7.50% (9.832% at June 30, 2019) and require quarterly principal and interest payments.

⁽¹⁸⁾	In January 2019, the Company financed certain flight equipment with $91.2 million in debt. The debt bears interest at the monthly LIBOR plus 3.10% (5.488% at June 30, 2019) and requires monthly principal and interest payments.

⁽¹⁹⁾	In June 2019, the Company financed ten CRJ-700 aircraft with $70.0 million in debt, which were previously leased. The debt bears interest at the monthly LIBOR plus 5.25% (7.638% at June 30, 2019) and requires monthly principal and interest payments.

Principal maturities of long-term debt as of June 30, 2019, and for each of the next five years are as follows (in thousands):

	Total Principal
Periods Ending September 30,	Amount
Remainder of 2019	$	45,182
2020		164,734
2021		159,839
2022		150,819
2023		101,247
Thereafter		274,909
	$	896,730

The net book value of collateralized aircraft and equipment as of June 30, 2019 was $1,203.0 million.

In December 2015, an Enhanced Equipment Trust Certificate ("EETC") pass-through trust was created to issue pass-through certificates to obtain financing for new E-175 aircraft. At June 30, 2019 Mesa has $200.3 million of equipment notes outstanding issued under the EETC financing included in long-term debt on the condensed consolidated balance sheets. The structure of the EETC financing consists of a pass-through trust created by Mesa to issue pass-through certificates, which represent fractional undivided interests in the pass-through trust and are not obligations of Mesa.

The proceeds of the issuance of the pass-through certificates were used to purchase equipment notes which were issued by Mesa and secured by its aircraft. The payment obligations under the equipment notes are those of Mesa. Proceeds received from the sale of pass-through certificates were initially held by a depositary in escrow for the benefit of the certificate holders until Mesa issued equipment notes to the trust, which purchased such notes with a portion of the escrowed funds.

Mesa evaluated whether the pass-through trust formed for its EETC financing is a Variable Interest Entity ("VIE") and required to be consolidated. The pass-through trust was determined to be a VIE, however, the Company has determined that it does not have a variable interest in the pass-through trust, and therefore, has not consolidated the pass-through trust with its financial statements.

On June 27, 2018, the Company refinanced $16.0 million of debt on six CRJ-900 aircraft with $27.5 million of debt, resulting in net cash proceeds to the Company of $10.4 million after transaction related fees. The notes payable require quarterly payments of principal and interest through fiscal 2022 bearing interest at LIBOR plus 3.50%.

On June 28, 2018, the Company purchased nine CRJ-900 aircraft, which were previously leased under its aircraft lease facility with Wells Fargo Bank Northwest, National Association, as owner trustee and lessor (the "GECAS Lease Facility"), for $76.5 million. The Company financed the aircraft purchase with $69.6 million in new debt and proceeds from the June 2018 refinancing of six CRJ-900 aircraft. The notes payable of $69.6 million require quarterly payments of principal and interest through fiscal 2022 bearing interest at LIBOR plus a spread ranging from 3.50% for the senior promissory notes to 7.50% for the subordinated promissory notes. The Company recorded non-cash lease termination expense of $15.1 million in connection with the lease buyout. Also, as part of the transaction, the Company (i) received $4.5 million of future goods and services credits and $5.6 million of loan forgiveness for loans with a maturity date in 2027 from the aircraft manufacturer, and (ii) mutually agreed with GE Capital Aviation Services LLC to terminate the GE Warrant to purchase 250,000 shares of common stock.

On August 14, 2018 the Company paid down the outstanding balance on the CIT Revolving Credit Facility of $25.7 million.

On January 28, 2019, the Company entered into a Term Loan Agreement (the "Term Loan") pursuant to which the lenders thereunder committed to lend to the Company term loans in the aggregate principal amount of $91.2 million. Borrowings under the Term Loan bear interest at LIBOR plus 3.10%. This interest rate is significantly lower than the interest rate under the Company's Spare Engine Facility (defined below), which the Term Loan refinanced and replaced. The Spare Engine Facility accrued interest at LIBOR plus 7.25%. The Term Loan has a term of five years, with principal and interest payments due monthly over the term of the loan in accordance with an amortization schedule. The Company recorded a loss on extinguishment of debt of $3.6 million, due to a $1.9 million write-off of financing fees and $1.7 million in prepayment penalties, in connection with the repayment of the Spare Engine Facility.

On June 14, 2019, the Company completed the purchase of ten CRJ-700 aircraft, which were previously leased under the GECAS Lease Facility, for $70.0 million. The Company financed the aircraft purchase with $70.0 million in new debt. The notes payable of $70.0 million require monthly payments of principal and interest through fiscal 2023 bearing interest at LIBOR plus 5.25%. The Company recorded non-cash lease termination expense of $9.5 million in connection with the lease buyout.

Earnings Per Share and Equity

9 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Earnings Per Share and Equity

9.	Earnings Per Share and Equity

Calculations of net income per common share attributable to Mesa Air Group were as follows (in thousands, except per share data):

	Three Months Ended June 30,					Nine Months Ended June 30,
	2019		2018			2019		2018
Net income attributable to Mesa Air Group	$	3,007	$	(11,135	)	$	35,337	$	13,861
Basic weighted average common shares outstanding		34,835		23,336			34,683		23,298
Add: Incremental shares for: (1)
Dilutive effect of warrants		—		—			—		140
Dilutive effect of restricted stock		277		—			368		334
Diluted weighted average common shares outstanding		35,112		23,336			35,051		23,772
Net income per common share attributable to Mesa Air Group:
Basic	$	0.09	$	(0.48	)	$	1.02	$	0.59
Diluted	$	0.09	$	(0.48	)	$	1.01	$	0.58

(1)	Due to a loss for the three months ended June 30, 2018, zero incremental shares are included because the effect would be antidilutive.

Basic income per common share is computed by dividing net income attributable to Mesa Air Group by the weighted average number of common shares outstanding during the period.

The number of incremental shares from the assumed issuance of shares relating to restricted stock and exercise of warrants (excluding warrants with a nominal conversion price) is calculated by applying the treasury stock method. Share-based awards and warrants whose impact is considered to be anti-dilutive under the treasury stock method were excluded from the diluted net income or loss per share calculation. In loss periods, these incremental shares are excluded from the calculation of diluted loss per share, as the inclusion of unvested restricted stock and warrants would have an anti-dilutive effect. Anti-dilutive shares relating to restricted stock and exercise of warrants of 418,954 and 160,139, respectively, were excluded from the calculation of diluted loss per share for the three months ended June 30, 2018.

Restatement

Basic and Diluted Net Income per Common Share Available to Common Shareholders for the Three Months and Nine Months Ended June 30, 2018

Subsequent to the issuance of the Company’s condensed consolidated financial statements for the three months ended December 31, 2018, we determined our previously issued condensed consolidated financial statements included warrants with a nominal conversion price in diluted income per common share available to common shareholders, however, they should have been included in basic income per common share available to common shareholders. Since diluted shares are under the treasury stock method, diluted income per common share available to common shareholders was impacted by this change. As a result, basic and diluted income per common share available to common shareholders for the periods ended June 30, 2018 have been restated to appropriately include the warrants with a nominal conversion price in basic income per common share available to common shareholders.

The restatement adjustment did not impact our previously reported consolidated balance sheet, consolidated statements of stockholders’ equity, consolidated statements of cash flows, or net income (in thousands, except per share data).

	Three Months Ended June 30, 2018						Nine Months Ended June 30, 2018
	As Previously Reported			As Restated			As Previously Reported		As Restated
Net income per common share available to common shareholders - Basic	$	(0.89	)	$	(0.48	)	$	1.18	$	0.59
Shares used in computing net income per common share available to common shareholders - Basic		12,462			23,336			11,782		23,298
Net income per common share available to common shareholders - Diluted	$	(0.89	)	$	(0.48	)	$	0.58	$	0.58
Shares used in computing net income per common share available to common shareholders - Diluted		12,462			23,336			24,052		23,772

Common Stock

9 Months Ended

Jun. 30, 2019

Stockholders Equity Note [Abstract]

Common Stock

10.	Common Stock

The Company previously issued warrants to third parties, which had a five-year term to be converted to common stock at an exercise price of $0.004 per share. Certain persons who are not U.S. citizens currently hold outstanding warrants to purchase shares of the Company's common stock. The warrants are exercisable if consistent with federal law, which requires that no more than 24.9% of the Company's stock be voted, directly or indirectly, or controlled by persons who are not U.S. citizens. The warrants can be converted to common stock upon warrant holders demonstrating U.S. citizenship or if consistent with above described federal law ownership limitations. In June 2018, the Company and holders agreed to extend the term of outstanding warrants set to expire by five years (through fiscal year 2023).

On June 28, 2018, the Company agreed with GE Capital Aviation Services LLC ("GE Capital") to terminate a warrant to purchase 250,000 shares of common stock held by GE Capital.

In July 2018, the Company's Board of Directors and Compensation Committee approved the issuance of shares of restricted common stock under its 2018 Equity Incentive Plan (the "2018 Plan") immediately following completion of the Company's IPO to certain of its employees and directors in exchange for the cancellation of existing restricted phantom stock units, unvested restricted shares and SARs. The shares of restricted common stock issued under the 2018 Plan in exchange for the cancellation of restricted phantom stock units, unvested restricted shares and SARs are subject to vesting on the same terms set forth in the prior vesting schedules and are not subject to acceleration in connection with the 2018 Plan issuances.

On April 9, 2019, and pursuant to Section 4.4 of the 2018 Plan in connection with the 2.5-for-1 stock split effected on August 8, 2018, the board of directors approved an increase in the number of shares authorized for issuance under the 2018 Plan by 1,000,000 shares of common stock.

The Company's shares of common stock were listed on The NASDAQ Global Select Market under the symbol "MESA" effective August 10, 2018. On August 14, 2018, the Company completed its IPO, in which it issued and sold 9,630,000 shares of common stock, no par value, at a public offering price of $12.00 per share (the "Firm Shares"). Additionally, in connection with the IPO, the Company granted the underwriters an option to purchase up to an additional 1,444,500 shares of common stock at the same price. On September 11, 2018, the Company closed the sale of 1,344,500 shares ("Option Shares") of its common stock, in connection with the partial exercise of the overallotment option granted to the underwriters in its IPO. Of the 1,344,500 Option Shares sold, 723,985 were purchased directly from the Company and the remaining 620,515 shares were purchased directly from the selling shareholders. The Firm Shares and Option Shares were sold to the public for a price of $12.00 per share. The aggregate gross proceeds to us from the IPO were approximately $124.2 million. The Company received $111.7 million in net proceeds after deducting $8.7 million of underwriting discounts and commissions and $3.8 million in offering costs.

The Company has not historically paid dividends on shares of its common stock. Additionally, the Company's aircraft lease facility (the "RASPRO" Lease Facility) with RASPRO Trust 2005, a pass-through trust and its aircraft lease facility with Wells Fargo Bank Northwest, National Association, as owner trustee and lessor (the "GECAS Lease Facility") each contain restrictions that limit the Company's ability to or prohibit it from paying dividends to holders of its common stock.

Income Taxes

9 Months Ended

Jun. 30, 2019

Income Tax Disclosure [Abstract]

Income Taxes

11.	Income Taxes

The Company's effective tax rate (ETR) from continuing operations was 22.2% and 23.6% for the three months and nine months ended June 30, 2019, respectively, and 23.9% and 228.2% for the three months and nine months ended June 30, 2018, respectively. The Company’s ETR during the three months and nine months ended June 30, 2019 were different from the federal statutory rate of 21% as a result of the vesting and exercise of stock options, state taxes, and changes in the valuation allowance against state net operating losses.

The Company’s ETR during the nine months ended June 30, 2018 was significantly different than the statutory tax rate primarily as a result of the comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (the "Tax Act") enacted by the U.S. government on December 22, 2017. The Tax Act made broad and complex changes to the U.S. tax code that affected the Company's fiscal year ended September 30, 2018, including but not limited to (1) reducing the U.S. federal corporate tax rate, (2) changing rules related to uses and limitations of NOL carryforwards created in tax years beginning after December 31, 2017, (3) eliminating the corporate alternative minimum tax ("AMT") and changing how existing AMT credits can be realized, and (4) altering bonus depreciation rules that will allow for full expensing of qualified property. As a result of the reducing the U.S. federal corporate tax rate, we recorded a decrease to our net deferred tax liabilities. We had a blended U.S. federal tax rate of 24.5% for the year ended September 30, 2018 based on the applicable tax rates before and after the Tax Act and the number of days in the year.

As of September 30, 2018, the Company had aggregate federal and state net operating loss carryforwards of approximately $415.1 million and $199.6 million, respectively, which expire in 2027-2037 and 2019-2038, respectively. Approximately $0.9 million of state net operating loss carryforwards are expiring in 2019.

Share-Based Compensation and Stock Repurchases

9 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation and Stock Repurchases

12.	Share-Based Compensation and Stock Repurchases

Restricted Stock

The restricted share activity for the nine months ended June 30, 2019 were summarized as follows:

				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Restricted shares unvested at September 30, 2018		1,250,625		$	9.59
Granted		306,843			8.89
Vested		(616,332	)		9.59
Forfeited		(22,995	)		12.00
Restricted shares unvested at June 30, 2019		918,141		$	9.29

As of June 30, 2019, there was $7.0 million, of total unrecognized compensation cost related to unvested share-based compensation arrangements. That cost is expected to be recognized over a weighted-average period of 1.5 years.

Compensation cost for share-based awards are recognized on a straight-line basis over the vesting period. Share-based compensation expense for the three months ended June 30, 2019 and 2018 was $1.5 million and $11.6 million, respectively, and for the nine months ended June 30, 2019 and 2018 was $4.3 million and $12.6 million, respectively. Share-based compensation expenses are recorded in general and administrative expenses in the condensed consolidated statements of operations.

The Company repurchased 169,724 shares of its common stock for $1.5 million to cover the income tax obligation on vested employee equity awards and warrant conversions during the nine months ended June 30, 2019. During the nine months ended June 30, 2018, the Company repurchased 29,710 shares of its common stock for $0.5 million to cover the income tax obligation on vested employee equity awards.

The Company has granted restricted stock awards (“RSAs”) and restricted stock units (“RSUs”) as part of its long-term incentive compensation to employees and non-employee members of the Board of Directors. RSAs and RSUs generally vest over a period of 3 to 5 years for employees and over one year for members of the Board of Directors. The restricted common stock underlying RSAs are deemed issued and outstanding upon grant, and carry the same voting rights of unrestricted outstanding common stock. The restricted common stock underlying RSUs are not deemed issued or outstanding upon grant, and do not carry any voting rights.

Commitments

9 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments

13.	Commitments

At June 30, 2019, the Company leased 18 aircraft under noncancelable operating leases with remaining terms of up to 4.8 years. The Company has the option to terminate certain leases at various times throughout the lease. The Company headquarters and other facility noncancelable operating leases have remaining terms of up to 6.4 years. The leases require the Company to pay all taxes, maintenance, insurance, and other operating expenses. Rental expense is recognized on a straight-line basis over the lease term, net of lessor rebates and other incentives. Aggregate rental expense under all operating aircraft, equipment and facility leases totaled approximately $18.1 million and $21.6 million for the three months ended June 30, 2019 and 2018, respectively, and $55.9 million and $62.8 million for the nine months ended June 30, 2019 and 2018, respectively.

Future minimum lease payments as of June 30, 2019, under noncancelable operating leases are as follows (in thousands):

Periods Ending September 30,	Aircraft		Other		Total
Remainder of 2019		6,265		1,098		7,363
2020		44,231		2,837		47,068
2021		44,314		1,665		45,979
2022		29,751		1,339		31,090
2023		12,418		1,308		13,726
Thereafter		11,849		2,704		14,553
Total	$	148,828	$	10,951	$	159,779

The majority of the Company's leased aircraft are leased through trusts that have a sole purpose to purchase, finance, and lease these aircraft to the Company; therefore, they meet the criteria of a variable interest entity. However, since these are single-owner trusts in which the Company does not participate, the Company is not at risk for losses and is not considered the primary beneficiary. Management believes that the Company's maximum exposure under these leases is the remaining lease payments.

Supplemental Disclosure

9 Months Ended

Jun. 30, 2019

Supplemental Cash Flow Elements [Abstract]

Supplemental Disclosure

15.	Supplemental Disclosure

The Company adopted ASU 2016-18 on a retrospective basis during the quarter ended December 31, 2018. The following is a reconciliation of the captions in the Condensed Consolidated Balance Sheets to the Condensed Consolidated Statements of Cash Flows (in thousands):

	June 30,		September 30,
	2019		2018
Condensed Consolidated Balance Sheets
Cash and cash equivalents	$	79,909	$	103,311
Restricted cash		3,647		3,823
Cash, cash equivalents, and restricted cash in Condensed Consolidated Statement of Cash Flows	$	83,556	$	107,134

The restricted cash balance primarily includes deposits in trust accounts to collateralize letters of credit and to fund workers' compensation claims, landing fees, and other business needs.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP") and include the accounts of the Company and its wholly owned operating subsidiaries. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification ("ASC") and Accounting Standards Update ("ASU") of the Financial Accounting Standards Board ("FASB"). All intercompany accounts and transactions have been eliminated in consolidation. These condensed consolidated financial statements should be read in conjunction with, the Company's audited consolidated financial statements and notes thereto as of and for the year ended September 30, 2018 included in the Company's Annual Report on Form 10-K for the year ended September 30, 2018 on file with the U.S. Securities and Exchange Commission (the "SEC"). Information and footnote disclosures normally included in financial statements have been condensed or omitted in these condensed consolidated financial statements pursuant to the rules and regulations of the SEC and GAAP. These condensed consolidated financial statements reflect all adjustments that, in the opinion of management, are necessary to present fairly the results of operations for the interim periods presented. The Company is an "emerging growth company," as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act,") and may remain an emerging growth company until the last day of its fiscal year following the fifth anniversary of the IPO, subject to specified conditions. The JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to "opt out" of such extended transition period, which means that when a standard is issued or revised and it has different application dates for public or private companies, the Company will be subject to the same new or revised accounting standards as other public companies that are not emerging growth companies.
Adoption of New Revenue Standard	Adoption of New Revenue Standard On October 1, 2018, the Company adopted ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) ("ASU 2014-09" or "ASC 606") using the modified retrospective method. See Note 3: "Recent Accounting Pronouncements" for more information. To conform to ASC 606, the Company modified its revenue recognition policy as described below.
Revenue Recognition, Contract Liabilities and Contract Assets	Revenue Recognition The Company recognizes revenue when the service is provided under its capacity purchase agreements. Under these agreements, the major airline partners generally pay a fixed monthly minimum amount per aircraft, plus certain additional amounts based upon the number of flights and block hours flown. The contracts also include reimbursement of certain costs incurred by the Company in performing flight services. These costs, known as "pass-through costs," may include passenger and hull insurance as well as aircraft property taxes. Additionally, for the E-175 aircraft owned by United, the capacity purchase agreement provides that United will reimburse the Company for heavy airframe and engine maintenance, landing gear, APUs and component maintenance. The Company also receives compensation under its capacity purchase agreements for heavy maintenance expenses at a fixed hourly rate or per aircraft rate for all aircraft in scheduled service other than the E-175 aircraft owned by United. The contracts also include a profit margin on certain reimbursable costs, as well as a profit margin, incentives and penalties based on certain operational benchmarks. The Company is eligible to receive incentive compensation upon the achievement of certain performance criteria defined in the capacity purchase agreements. At the end of each period during the term of an agreement, the Company calculates the incentives achieved during that period and recognizes revenue attributable to the agreement during the period accordingly, subject to the variable constraint guidance under ASC 606. All revenue recognized under these contracts is presented as the gross amount billed to the major airline partners. Under the capacity purchase agreements, the Company has committed to perform various activities that can be generally classified into in-flight services and maintenance services. When evaluating these services, the Company determined that the nature of its promise is to provide a single integrated service, flight services, because its contracts require integration and assumption of risk associated with both services to effectively deliver and provide the flights as scheduled over the contract term. Therefore, the in-flight services and maintenance services are inputs to that combined integrated flight service. Both the services occur over the term of the agreement and the performance of maintenance services significantly effects the utility of the in-flight services. The Company's individual flights flown under the capacity purchase agreements are deemed to be distinct and the flight service promised in the capacity purchase agreements represents a series of services that should be accounted for as a single performance obligation. This single performance obligation is satisfied over time as the flights are completed. Therefore, revenue is recognized when each flight is completed. In allocating the transaction price, variable payments (i.e. billings based on flights and block hours flown, pass-through costs, etc.) that relate specifically to the Company's efforts in performing flight services are recognized in the period in which the individual flight is completed. The Company has concluded that allocating the variability directly to the individual flights results in an overall allocation meeting the objectives in ASC 606. This results in a pattern of revenue recognition that follows the variable amounts billed from the Company to their customers. A portion of the Company's compensation under its capacity purchase agreements with American and United is designed to reimburse the Company for certain aircraft ownership costs. The Company has concluded that a component of its revenue under these agreements is deemed to be lease revenue, as such agreements identify the "right of use" of a specific type and number of aircraft over a stated period-of-time. The lease revenue associated with the Company's capacity purchase agreements is accounted for as an operating lease and is reflected as contract revenue on the Company's condensed consolidated statements of operations. The Company recognized $54.5 million and $53.5 million of lease revenue for the three months ended June 30, 2019 and 2018, respectively, and $164.5 million and $162.0 million of lease revenue for the nine months ended June 30, 2019 and 2018, respectively. The Company has not separately stated aircraft rental income and aircraft rental expense in the condensed consolidated statements of operations because the use of the aircraft is not a separate activity of the total service provided. The Company's capacity purchase agreements are renewable periodically and contain provisions pursuant to which the parties could terminate their respective agreements, subject to certain conditions as described in Note 1. The capacity purchase agreements also contain terms with respect to covered aircraft, services provided and compensation as described in Note 1. The capacity purchase agreements are amended from time to time to change, add or delete terms of the agreements. The Company's revenues could be impacted by a number of factors, including amendment or termination of its capacity purchase agreements, contract modifications resulting from contract renegotiations, its ability to earn incentive payments contemplated under applicable agreements, and settlement of reimbursement disputes with the Company's major airline partners. In the event contracted rates are not finalized at a quarterly or annual financial statement date, the Company evaluates the enforceability of its contractual terms and when it has an enforceable right, it estimates the amount the Company expects to be entitled to that is subject to the ASC 606 constraint. The Company's capacity purchase agreements contain an option that allows its major airline partners to assume the contractual responsibility for procuring and providing the fuel necessary to operate the flights that it operates for them. Both of the Company's major airline partners have exercised this option. Accordingly, the Company does not record an expense or revenue for fuel and related fueling costs for flying under its capacity purchase agreements. In addition, the Company's major airline partners also provide, at no cost to the Company, certain ground handling and customer service functions, as well as airport-related facilities and gates at their hubs and other cities. Services and facilities provided by the Company's major airline partners at no cost are presented net in its condensed consolidated financial statements; hence, no amounts are recorded for revenue or expense for these items. Contract Liabilities Contract liabilities consist of deferred credits for cost reimbursements from major airline partners related to aircraft modifications associated with capacity purchase agreements and pilot training. The deferred credits are recognized over time depicting the pattern of transfer of control of services resulting in ratable recognition of revenue over the remaining term of the capacity purchase agreements. Current and non-current deferred credits are recorded to other accrued expenses and non-current deferred credits in the condensed consolidated balance sheets. The Company's total current and non-current deferred credit balances at June 30, 2019 and September 30, 2018 are $13.4 million and $15.4 million, respectively. The Company recognized $1.3 million and $1.0 million of the deferred credits to revenue in the condensed consolidated statements of operations during the three months ended June 30, 2019 and 2018, respectively, and $3.8 million and $3.2 million during the nine months ended June 30, 2019 and 2018, respectively. Contract Assets The Company recognizes assets from the costs incurred to fulfill a contract including aircraft painting and reconfiguration and flight service personnel training costs. These costs are amortized based on the pattern of transfer of the services in relation to flight hours over the term of the contract. Contract assets are recorded as other assets in the condensed consolidated balance sheets. The Company's contract assets balances at June 30, 2019 and September 30, 2018 are $4.5 million and $4.6 million, respectively. Contract cost amortization was $0.6 million and $0.5 million for the three months ended June 30, 2019 and 2018, respectively, and $1.8 million and $1.4 million for the nine months ended June 30, 2019 and 2018, respectively.
Use of Estimates	Use of Estimates The preparation of the Company's condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues, and expenses and the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements. Actual results could differ from those estimates.
Maintenance Expense	Maintenance Expense The Company operates under a Federal Aviation Administration ("FAA") approved continuous inspection and maintenance program. The Company uses the direct expense method of accounting for its maintenance of regional jet engine overhauls, airframe, landing gear, and normal recurring maintenance wherein the Company recognizes the expense when the maintenance work is completed, or over the repair period, if materially different. For leased aircraft, the Company is subject to lease return provisions that require a minimum portion of the "life" of an overhaul be remaining on the engine at the lease return date. The Company estimates the cost of maintenance lease return obligations and accrues such costs over the remaining lease term when the expense is probable and can be reasonably estimated. Under the Company's aircraft operating lease agreements and FAA operating regulations, it is obligated to perform all required maintenance activities on its fleet, including component repairs, scheduled air frame checks and major engine restoration events. The Company estimates the timing of the next major maintenance event based on assumptions including estimated usage, FAA-mandated maintenance intervals and average removal times as recommended by the manufacturer. The timing and the cost of maintenance are based on estimates, which can be impacted by changes in utilization of its aircraft, changes in government regulations and suggested manufacturer maintenance intervals. Major maintenance events consist of overhauls to major components. Engine overhaul expense totaled $11.4 million and $13.1 million for the three months ended June 30, 2019, and 2018, respectively, of which $1.9 million and $4.6 million, respectively, was pass-through expense. Engine overhaul expense totaled $20.7 million and $46.2 million for the nine months ended June 30, 2019, and 2018, respectively, of which $3.0 million and $9.8 million, respectively, was pass-through expense. Airframe C-check expense totaled $4.4 million and $6.3 million for the three months ended June 30, 2019, and 2018, respectively, of which $0 million and $1.6 million, respectively, was pass-through expense. Airframe C-check expense totaled $10.6 million and $19.8 million for the nine months ended June 30, 2019, and 2018, respectively, of which $0 million and $7.3 million, respectively, was pass-through expense.
Change in Accounting Policy	Change in Accounting Policy Stock Appreciation Rights (“SARs”) and Phantom Stock historically were accounted for as liability compensatory awards under ASC 710, Compensation – General, valued using the intrinsic value method, as permitted by ASC 718, Compensation – Stock Compensation, for nonpublic entities. Upon becoming a public company, as defined in ASC 718, in the third quarter of fiscal 2018, the Company was required to change its methodology for valuing the SARs and Phantom Stock. The SARs and Phantom Stock were re-measured at each quarterly reporting date and were accounted for prospectively at fair value using a Black-Scholes fair value pricing model until they were converted to restricted stock awards upon completion of the Company’s IPO. The Company recorded the impact of the change in valuation methods as a cumulative effect of a change in accounting principle, as permitted by ASC 250, Accounting Changes and Error Corrections. The effect of the change increased the SARs and Phantom Stock liability by $2.4 million which was the difference in compensation cost measured using the intrinsic value method and the fair value method. An equal and offsetting change to retained earnings in the consolidated balance sheet was recorded with the revaluation. Any future changes in fair value were recorded as compensation expense in the consolidated statement of operations. Upon completion of the Company’s IPO the SARs and Phantom Stock were cancelled and exchanged for shares of restricted stock under our 2018 Plan.

Intangible Assets (Tables)

9 Months Ended

Jun. 30, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Information About Intangible Assets

Information about the intangible assets of the Company at June 30, 2019 and September 30, 2018, were as follows (in thousands):

	June 30,			September 30,
	2019			2018
Customer relationship	$	43,800		$	43,800
Accumulated amortization		(33,816	)		(32,459	)
	$	9,984		$	11,341

Balance Sheet Information (Tables)

9 Months Ended

Jun. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Summary of Certain Significant Amounts Included in Condensed Consolidated Balance Sheet

Certain significant amounts included in the Company's condensed consolidated balance sheet as of June 30, 2019 and September 30, 2018, consisted of the following (in thousands):

	June 30,			September 30,
	2019			2018
Expendable parts and supplies, net:
Expendable parts and supplies	$	23,957		$	18,907
Less: obsolescence and other		(3,689	)		(3,249	)
	$	20,268		$	15,658

Prepaid expenses and other current assets:
Prepaid aircraft rent	$	39,785		$	30,267
Unutilized manufacturer credits		1,500			4,500
Deferred offering and reimbursed costs		2,090			1,945
Other		3,050			4,202
	$	46,425		$	40,914
Property and equipment, net:
Aircraft and other flight equipment substantially pledged	$	1,577,052		$	1,502,940
Other equipment		4,757			3,721
Leasehold improvements		2,797			2,754
Vehicles		924			692
Building		699			699
Furniture and fixtures		291			287
Total property and equipment		1,586,520			1,511,093
Less: accumulated depreciation		(300,498	)		(260,264	)
	$	1,286,022		$	1,250,829

Other accrued expenses:
Accrued property taxes	$	7,667		$	6,981
Accrued interest		7,729			6,118
Accrued vacation		5,894			5,470
Accrued wheels, brakes and tires		1,567			1,452
Other		11,295			9,675
	$	34,152		$	29,696

Fair Value Measurements (Tables)

9 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Carrying Value and Estimated Fair Value of Long-term Debt, Including Current Maturities

The carrying value and estimated fair value of the Company's long-term debt, including current maturities, were as follows (in thousands):

	June 30, 2019				September 30, 2018
	Carrying		Fair		Carrying		Fair
	Value		Value		Value		Value
Long-term debt, including current maturities⁽¹⁾	$	896,730	$	917,265	$	930,207	$	926,229

⁽¹⁾	Current and prior period long-term debts' carrying and fair values exclude net debt issuance costs.

Long-Term Debt and Other Borrowings (Tables)

9 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Schedule of Long-term Debt

Long-term debt as of June 30, 2019 and September 30, 2018, consisted of the following (in thousands):

	June 30,			September 30,
	2019			2018
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2019⁽¹⁾⁽²⁾	$	—		$	4,428
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2022⁽³⁾⁽⁴⁾		54,513			69,340
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2024⁽⁵⁾		64,090			72,438
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2027⁽⁶⁾		113,861			122,591
Notes payable to secured parties, collateralized by the underlying
aircraft, due 2028⁽⁷⁾		200,322			209,240
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2028⁽⁸⁾		156,573			167,269
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2022⁽¹⁷⁾		77,910			95,060
Notes payable to financial institution, collateralized by the underlying
equipment, due 2022⁽⁹⁾	—				88,162
Senior and subordinated notes payable to secured parties, collateralized
by the underlying aircraft, due 2022⁽¹⁰⁾		51,421			63,403
Notes payable to financial institution, collateralized by the underlying
equipment, due 2020⁽¹¹⁾		2,073			3,318
Notes payable to financial institution due 2020⁽¹²⁾		2,837			4,360
Notes payable to financial institution, collateralized by the underlying
equipment, due 2020⁽¹³⁾		9,753			14,971
Notes payable to financial institution due 2019⁽¹⁴⁾		—			5,896
Working capital draw loan, collateralized by certain flight equipment
and spare parts⁽¹⁵⁾		—			—
Other obligations due to financial institution, collateralized by the underlying
equipment, due 2023⁽¹⁶⁾		8,840			9,731
Notes payable to financial institution, collateralized by the underlying
equipment, due 2024⁽¹⁸⁾		84,537			—
Notes payable to financial institution, collateralized by the underlying
aircraft, due 2023⁽¹⁹⁾		70,000			—
Total long-term debt		896,730			930,207
Less current portion		(163,623	)		(155,170	)
Less unamortized debt issuance costs		(15,561	)		(14,860	)
Long-term debt—excluding current portion	$	717,546		$	760,177

⁽¹⁾	In fiscal 2005, the Company financed five CRJ-900 aircraft with $118.0 million in debt. The debt bears interest at the monthly London Inter-bank Offered Rate ("LIBOR"), plus 3.00% and requires monthly principal and interest payments.

⁽²⁾	In fiscal 2004, the Company financed five CRJ-700 and nine CRJ-900 aircraft with $254.7 million in debt. The debt bears interest at the monthly LIBOR plus 3.00% and requires monthly principal and interest payments.

⁽³⁾	In fiscal 2007, the Company financed three CRJ-900 and three CRJ-700 aircraft for $120.3 million. The debt bears interest at the monthly LIBOR plus 2.25% (4.638% at June 30, 2019) and requires monthly principal and interest payments.

⁽⁴⁾	In fiscal 2014, the Company financed 10 CRJ-900 aircraft for $88.4 million. The debt bears interest at the monthly LIBOR plus a spread ranging from 1.95% to 7.25% (4.338% to 9.638% at June 30, 2019) and requires monthly principal and interest payments.

⁽⁵⁾	In fiscal 2014, the Company financed eight CRJ-900 aircraft with $114.5 million in debt. The debt bears interest at 5.00% and requires monthly principal and interest payments.

⁽⁶⁾

⁽⁷⁾	In fiscal 2016, the Company financed 10 E-175 aircraft with $246.0 million in debt under an EETC financing arrangement (see discussion below). The debt bears interest ranging from 4.75% to 6.25% and requires semi-annual principal and interest payments.

⁽⁸⁾

⁽⁹⁾	In fiscal 2017, the Company financed certain flight equipment with $99.1 million in debt. The debt bears interest at the monthly LIBOR (rounded to the nearest 16th) plus 7.25% (9.638% at June 30, 2019) and requires monthly principal and interest payments.

⁽¹⁰⁾

⁽¹¹⁾	In fiscal 2015, the Company financed certain flight equipment with $8.3 million in debt. The debt bears interest at 5.163% and requires monthly principal and interest payments.

⁽¹²⁾	In fiscal 2015 and 2016, the Company financed certain flight equipment maintenance costs with $10.2 million in debt. The debt bears interest at the monthly LIBOR plus 3.07% (5.402% at June 30, 2019) and requires quarterly principal and interest payments.

⁽¹³⁾

⁽¹⁴⁾

⁽¹⁵⁾

⁽¹⁶⁾	In February 2018, the Company leased two spare engines. The leases were determined to be capital as the leases contain a bargain purchase option at the end of the term. Imputed interest is 9.128% and the leases requires monthly payments.

⁽¹⁷⁾

⁽¹⁸⁾	In January 2019, the Company financed certain flight equipment with $91.2 million in debt. The debt bears interest at the monthly LIBOR plus 3.10% (5.488% at June 30, 2019) and requires monthly principal and interest payments.

⁽¹⁹⁾	In June 2019, the Company financed ten CRJ-700 aircraft with $70.0 million in debt, which were previously leased. The debt bears interest at the monthly LIBOR plus 5.25% (7.638% at June 30, 2019) and requires monthly principal and interest payments.

Schedule of Principal Maturities of Long-term Debt

Principal maturities of long-term debt as of June 30, 2019, and for each of the next five years are as follows (in thousands):

	Total Principal
Periods Ending September 30,	Amount
Remainder of 2019	$	45,182
2020		164,734
2021		159,839
2022		150,819
2023		101,247
Thereafter		274,909
	$	896,730

Earnings Per Share and Equity (Tables)

9 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Calculations of Net Income Per Common Share

Calculations of net income per common share attributable to Mesa Air Group were as follows (in thousands, except per share data):

	Three Months Ended June 30,					Nine Months Ended June 30,
	2019		2018			2019		2018
Net income attributable to Mesa Air Group	$	3,007	$	(11,135	)	$	35,337	$	13,861
Basic weighted average common shares outstanding		34,835		23,336			34,683		23,298
Add: Incremental shares for: (1)
Dilutive effect of warrants		—		—			—		140
Dilutive effect of restricted stock		277		—			368		334
Diluted weighted average common shares outstanding		35,112		23,336			35,051		23,772
Net income per common share attributable to Mesa Air Group:
Basic	$	0.09	$	(0.48	)	$	1.02	$	0.59
Diluted	$	0.09	$	(0.48	)	$	1.01	$	0.58

(1)	Due to a loss for the three months ended June 30, 2018, zero incremental shares are included because the effect would be antidilutive.

Summary of Basic and Diluted Net Income per Common Share Available to Common Shareholders

	Three Months Ended June 30, 2018						Nine Months Ended June 30, 2018
	As Previously Reported			As Restated			As Previously Reported		As Restated
Net income per common share available to common shareholders - Basic	$	(0.89	)	$	(0.48	)	$	1.18	$	0.59
Shares used in computing net income per common share available to common shareholders - Basic		12,462			23,336			11,782		23,298
Net income per common share available to common shareholders - Diluted	$	(0.89	)	$	(0.48	)	$	0.58	$	0.58
Shares used in computing net income per common share available to common shareholders - Diluted		12,462			23,336			24,052		23,772

Share-Based Compensation and Stock Repurchases (Tables)

9 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Restricted Share Activity

The restricted share activity for the nine months ended June 30, 2019 were summarized as follows:

				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Restricted shares unvested at September 30, 2018		1,250,625		$	9.59
Granted		306,843			8.89
Vested		(616,332	)		9.59
Forfeited		(22,995	)		12.00
Restricted shares unvested at June 30, 2019		918,141		$	9.29

Commitments (Tables)

9 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

Future minimum lease payments as of June 30, 2019, under noncancelable operating leases are as follows (in thousands):

Periods Ending September 30,	Aircraft		Other		Total
Remainder of 2019		6,265		1,098		7,363
2020		44,231		2,837		47,068
2021		44,314		1,665		45,979
2022		29,751		1,339		31,090
2023		12,418		1,308		13,726
Thereafter		11,849		2,704		14,553
Total	$	148,828	$	10,951	$	159,779

Supplemental Disclosure (Tables)

9 Months Ended

Jun. 30, 2019

Supplemental Cash Flow Elements [Abstract]

Reconciliation of Captions in Condensed Consolidated Balance Sheets to Condensed Consolidated Statements of Cash Flows

The following is a reconciliation of the captions in the Condensed Consolidated Balance Sheets to the Condensed Consolidated Statements of Cash Flows (in thousands):

	June 30,		September 30,
	2019		2018
Condensed Consolidated Balance Sheets
Cash and cash equivalents	$	79,909	$	103,311
Restricted cash		3,647		3,823
Cash, cash equivalents, and restricted cash in Condensed Consolidated Statement of Cash Flows	$	83,556	$	107,134

Organization and Operations - Additional Information (Detail) $ / shares in Units, $ in Millions							1 Months Ended	9 Months Ended	12 Months Ended
	Aug. 12, 2019 Air-craft Extension	Jun. 30, 2019 Daily_Departure State Air-craft shares	Apr. 09, 2019	Sep. 11, 2018 $ / shares shares	Aug. 14, 2018 USD ($) $ / shares shares	Aug. 08, 2018 shares	Jul. 31, 2019 Air-craft	Jun. 30, 2019 USD ($) Daily_Departure State Air-craft City Employee Extension shares	Sep. 30, 2018 Air-craft Extension shares	Apr. 02, 2019 Air-craft
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of states in which entity operates \| State		42						42
Number of aircrafts operated \| Air-craft								145
Number of cities in which entity operates \| City								138
Number of daily departures \| Daily_Departure		761						761
Number of employees \| Employee								3,400
Common stock, stock split								Effected a 2.5-for-1 stock split
Stock split ratio						2.5
Common stock, shares authorized		125,000,000				125,000,000		125,000,000	125,000,000
Preferred stock, shares authorized		5,000,000				5,000,000		5,000,000	5,000,000
Stock issued during period, shares, new issues				1,344,500
Common stock, warrants issued		4,603,333						4,603,333	10,614,990
Maximum number of aircraft to permanently withdraw in the event company fails to meet new/ revise performance metrics \| Air-craft		6
Number of aircraft converted to be utilized as operational spares \| Air-craft										2
Notice period for termination of agreement								The capacity purchase agreement is also subject to early termination for cause under specified circumstances and subject to the Company's right to cure under certain circumstances. United is also permitted, subject to certain conditions, to terminate the agreement early in its discretion by giving Mesa notice of 90 days or more
United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of extension \| Extension								4	4
Renewal term extension period								2 years	2 years
Subsequent Event [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircraft permanently withdraw due to failure to meet new/revised performance metrics \| Air-craft							2
Minimum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Capacity purchase agreement, expiration year								2021
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Capacity purchase agreement, expiration year								2025
Maximum [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Aggregate extension period								8 years	8 years
CRJ-900 Aircraft [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircrafts operated \| Air-craft								62
CRJ-700 Aircraft [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircrafts operated \| Air-craft								20
CRJ-700 Aircraft [Member] \| Minimum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period								2019-08
CRJ-700 Aircraft [Member] \| Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period								2019-12
E-175 Aircraft [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircrafts operated \| Air-craft								60
E-175 Aircraft [Member] \| Mesa [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircrafts operated \| Air-craft								18
E-175 Aircraft [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircrafts operated \| Air-craft								30	12
E-175 Aircraft [Member] \| Subsequent Event [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of aircrafts operated \| Air-craft	30
Number of extension \| Extension	4
Renewal term extension period	2 years
E-175 Aircraft [Member] \| Minimum [Member] \| Mesa [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period								2028-01
E-175 Aircraft [Member] \| Minimum [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period								2019-06
E-175 Aircraft [Member] \| Minimum [Member] \| Subsequent Event [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period	2019-10
E-175 Aircraft [Member] \| Maximum [Member] \| Mesa [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period								2028-11
E-175 Aircraft [Member] \| Maximum [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period								2020-08
E-175 Aircraft [Member] \| Maximum [Member] \| Subsequent Event [Member] \| United [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Early termination right, expiration period	2020-08
Aggregate extension period	8 years
American Airlines Inc. [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Ownership interest		7.20%						7.20%	7.20%
Restricted Stock [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of Shares, Granted								306,843
2018 Equity Incentive Plan [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock, stock split								2.5-for-1 stock split effected
Stock split ratio			2.5
Stock withheld to cover tax withholding requirements upon vesting to restricted stock units shares					314,198
Restricted stock, shares issued net of shares for tax withholdings					651,824
2018 Equity Incentive Plan [Member] \| Restricted Stock [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of Shares, Granted					1,266,034
Number of Shares, vested					966,022
2018 Equity Incentive Plan [Member] \| Restricted Stock [Member] \| Employees [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Additional restricted common stock shares issued					983,113
2011 and 2017 Plans [Member] \| Restricted Stock [Member] \| Employees [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Cancellation of common stock shares existing unvested restricted phantom units					491,915
Cancellation of common stock shares existing of unvested restricted stock					491,198
IPO [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period, shares, new issues					9,630,000
Shares issued, price per share \| $ / shares					$ 12.00
Gross proceeds of initial public offering \| $					$ 115.6			$ 124.2
Net proceeds from initial public offering \| $								111.7
Underwriting discounts and commissions \| $								8.7
Offering costs \| $								$ 3.8
Underwriter Options [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock, warrants issued					1,444,500
Purchased Directly From Company [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period, shares, new issues				723,985
Purchased Directly From Selling Shareholders [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period, shares, new issues				620,515
Firm Shares and Option Shares [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued, price per share \| $ / shares				$ 12.00

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Accounting Policies [Abstract]
Lease revenue	$ 54.5	$ 53.5	$ 164.5	$ 162.0
Current and non-current deferred credit balances	13.4		13.4		$ 15.4
Recognized deferred credits to revenue	1.3	1.0	3.8	3.2
Contract assets balances	4.5		4.5		$ 4.6
Contract cost amortization	0.6	0.5	1.8	1.4
Engine overhaul expense	11.4	13.1	20.7	46.2
Engine overhaul pass-through expense	1.9	4.6	3.0	9.8
Airframe check expense	4.4	6.3	10.6	19.8
Airframe check pass-through expense	$ 0.0	$ 1.6	0.0	$ 7.3
Change in accounting policy , effect of increase in SARs and Phantom Stock liability			$ 2.4

Concentrations - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Concentration Risk [Line Items]
Allowance for doubtful accounts	$ 1.2		$ 1.2		$ 1.3
Sales Revenue, Net [Member] \| American Airlines Inc. [Member] \| Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	53.00%	54.00%	54.00%	57.00%
Sales Revenue, Net [Member] \| United [Member] \| Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	47.00%	46.00%	46.00%	43.00%

Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Intangible Assets Net Excluding Goodwill [Abstract]
Amortization expense recognized	$ 0.5	$ 0.1	$ 1.4	$ 0.3
Amortization expense for remainder of 2019	0.5		0.5
Amortization expense for 2020	1.5		1.5
Amortization expense for 2021	1.2		1.2
Amortization expense for 2022	1.0		1.0
Amortization expense for 2023	$ 0.9		$ 0.9

Balance Sheet Information - Summary of Certain Significant Amounts Included in Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Expendable parts and supplies, net:
Expendable parts and supplies	$ 23,957	$ 18,907
Less: obsolescence and other	(3,689)	(3,249)
Expendable parts and supplies, net	20,268	15,658
Prepaid expenses and other current assets:
Prepaid aircraft rent	39,785	30,267
Unutilized manufacturer credits	1,500	4,500
Deferred offering and reimbursed costs	2,090	1,945
Other	3,050	4,202
Prepaid expenses and other current assets	46,425	40,914
Property and equipment, net:
Property and equipment-gross	1,586,520	1,511,093
Less: accumulated depreciation	(300,498)	(260,264)
Property and equipment-net	1,286,022	1,250,829
Other accrued expenses:
Accrued property taxes	7,667	6,981
Accrued interest	7,729	6,118
Accrued vacation	5,894	5,470
Accrued wheels, brakes and tires	1,567	1,452
Other	11,295	9,675
Other accrued expenses	34,152	29,696
Aircraft and Other Flight Equipment Substantially Pledged [Member]
Property and equipment, net:
Property and equipment-gross	1,577,052	1,502,940
Other Machinery and Equipment [Member]
Property and equipment, net:
Property and equipment-gross	4,757	3,721
Leasehold Improvements [Member]
Property and equipment, net:
Property and equipment-gross	2,797	2,754
Vehicles [Member]
Property and equipment, net:
Property and equipment-gross	924	692
Building [Member]
Property and equipment, net:
Property and equipment-gross	699	699
Furniture and Fixtures [Member]
Property and equipment, net:
Property and equipment-gross	$ 291	$ 287

Fair Value Measurements - Carrying Value and Estimated Fair Value of Long-term Debt, Including Current Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Long-term debt, including current maturities, carrying value	$ 896,730	$ 930,207
Long-term debt, including current maturities, fair value	$ 917,265	$ 926,229

Long-Term Debt and Other Borrowings - Schedule of Long-term Debt (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Total long-term debt	$ 896,730	$ 930,207
Less current portion	(163,623)	(155,170)
Less unamortized debt issuance costs	(15,561)	(14,860)
Long-term debt—excluding current portion	717,546	760,177
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member]
Debt Instrument [Line Items]
Total long-term debt		4,428
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Total long-term debt	54,513	69,340
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2024 [Member]
Debt Instrument [Line Items]
Total long-term debt	64,090	72,438
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2027 [Member]
Debt Instrument [Line Items]
Total long-term debt	113,861	122,591
Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Total long-term debt	200,322	209,240
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Total long-term debt	156,573	167,269
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Total long-term debt	77,910	95,060
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Total long-term debt	51,421	63,403
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member]
Debt Instrument [Line Items]
Total long-term debt		88,162
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment [Member]
Debt Instrument [Line Items]
Total long-term debt	2,073	3,318
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Total long-term debt	9,753	14,971
Notes Payable to Financial Institution Due 2020 [Member]
Debt Instrument [Line Items]
Total long-term debt	2,837	4,360
Notes Payable to Financial Institution Due 2019 [Member]
Debt Instrument [Line Items]
Total long-term debt		5,896
Other Obligations Due to Financial Institution, Collateralized by the Underlying Equipment, Due 2023 [Member]
Debt Instrument [Line Items]
Total long-term debt	8,840	$ 9,731
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2024 [Member]
Debt Instrument [Line Items]
Total long-term debt	84,537
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2023 [Member]
Debt Instrument [Line Items]
Total long-term debt	$ 70,000

Long-Term Debt and Other Borrowings - Schedule of Principal Maturities of Long-term Debt (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Long Term Debt By Maturity [Abstract]
Remainder of 2019	$ 45,182
2020	164,734
2021	159,839
2022	150,819
2023	101,247
Thereafter	274,909
Total long-term debt	$ 896,730	$ 930,207

Long-Term Debt and Other Borrowings - Additional Information (Detail) $ in Thousands						1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 14, 2019 USD ($) Air-craft	Jan. 28, 2019 USD ($)	Aug. 14, 2018 USD ($)	Jun. 28, 2018 USD ($) Air-craft shares	Jun. 27, 2018 USD ($) Air-craft	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Debt Instrument [Line Items]
Property and equipment, net							$ 1,286,022		$ 1,286,022		$ 1,250,829
Long-Term Debt							896,730		896,730		$ 930,207
Non-cash lease termination expense							$ 9,540	$ 15,109	$ 9,540	$ 15,109
Warrants to purchase shares of common stock \| shares							250,000		250,000
CIT revolving credit facility outstanding			$ 25,700
Loss on extinguishment of debt									$ 3,616
Payment for debt extinguishment prepayment penalties									1,672
CRJ-900 [Member]
Debt Instrument [Line Items]
Refinanced debt					$ 16,000
Number of aircraft under refinanced debt \| Air-craft				6	6
Debt instrument, face amount				$ 69,600	$ 27,500
Net cash proceeds after transaction related fees					$ 10,400
Long term debt interest rate description				LIBOR plus a spread ranging from 3.50% for the senior promissory notes to 7.50% for the subordinated promissory notes	LIBOR plus 3.50%.
Long term debt, basis spread on variable rate					3.50%
Number of aircraft purchased \| Air-craft				9
Payment for aircraft purchased				$ 76,500
Number of aircraft financed \| Air-craft				6	6
Non-cash lease termination expense				$ 15,100
Future goods and services credits				4,500
Loan forgiveness for loans				$ 5,600
Warrants to purchase shares of common stock \| shares				250,000
CRJ-700 Aircraft [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 70,000
Long term debt interest rate description	LIBOR plus 5.25%.
Long term debt, basis spread on variable rate	5.25%
Number of aircraft purchased \| Air-craft	10
Payment for aircraft purchased	$ 70,000
Non-cash lease termination expense	$ 9,500
Equipment Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt							$ 200,300		$ 200,300
Senior Notes Due 2020 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 65,800		65,800		65,800
Long term debt, basis spread on variable rate				3.50%		3.50%
Senior Notes Due 2020 [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate description									three-month LIBOR plus 3.50%
Subordinated Notes Due 2020 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 29,800		$ 29,800		$ 29,800
Long term debt, basis spread on variable rate				7.50%		7.50%
Subordinated Notes Due 2020 [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate description									three month LIBOR plus 7.50%
Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount		$ 91,200
Long term debt interest rate description		LIBOR plus 3.10%
Term loan, term		5 years
Write-off of financing cost		$ 1,900
Payment for debt extinguishment prepayment penalties		$ 1,700
Term Loan \| Spare Engine Facility [Member]
Debt Instrument [Line Items]
Long term debt interest rate description		LIBOR plus 7.25%
Loss on extinguishment of debt		$ 3,600
Term Loan \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt, basis spread on variable rate		3.10%
Term Loan \| LIBOR [Member] \| Spare Engine Facility [Member]
Debt Instrument [Line Items]
Long term debt, basis spread on variable rate		7.25%
Aircraft and Equipment [Member] \| Pledged as Collateral [Member]
Debt Instrument [Line Items]
Property and equipment, net							$ 1,203,000		$ 1,203,000

Earnings Per Share and Equity - Calculations of Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share And Equity [Abstract]
Net income	$ 3,007	$ 13,249	$ 19,081	$ (11,135)	$ 2,372	$ 22,624	$ 35,337	$ 13,861
Basic weighted average common shares outstanding	34,835,000			23,336,000			34,683,000	23,298,000
Add: Incremental shares for:
Dilutive effect of warrants				0				140,000
Dilutive effect of restricted stock	277,000						368,000	334,000
Diluted weighted average common shares outstanding	35,112,000			23,336,000			35,051,000	23,772,000
Net income per common share attributable to Mesa Air Group:
Basic	$ 0.09			$ (0.48)			$ 1.02	$ 0.59
Diluted	$ 0.09			$ (0.48)			$ 1.01	$ 0.58

Earnings Per Share and Equity - Summary of Basic and Diluted Net Income per Common Share Available to Common Shareholders (Detail) - $ / shares shares in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Line Items]
Net income per common share available to common shareholders - Basic	$ 0.09	$ (0.48)	$ 1.02	$ 0.59
Shares used in computing net income per common share available to common shareholders - Basic	34,835	23,336	34,683	23,298
Net income per common share available to common shareholders - Diluted	$ 0.09	$ (0.48)	$ 1.01	$ 0.58
Shares used in computing net income per common share available to common shareholders - Diluted	35,112	23,336	35,051	23,772
As Previously Reported
Earnings Per Share [Line Items]
Net income per common share available to common shareholders - Basic		$ (0.89)		$ 1.18
Shares used in computing net income per common share available to common shareholders - Basic		12,462		11,782
Net income per common share available to common shareholders - Diluted		$ (0.89)		$ 0.58
Shares used in computing net income per common share available to common shareholders - Diluted		12,462		24,052
As Restated
Earnings Per Share [Line Items]
Net income per common share available to common shareholders - Basic		$ (0.48)		$ 0.59
Shares used in computing net income per common share available to common shareholders - Basic		23,336		23,298
Net income per common share available to common shareholders - Diluted		$ (0.48)		$ 0.58
Shares used in computing net income per common share available to common shareholders - Diluted		23,336		23,772

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e1337-109256

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 55
-Paragraph 52
-URI http://asc.fasb.org/extlink&oid=109256448&loc=d3e4984-109258

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e1252-109256

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109260490&loc=SL5780133-109256

+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 55
-Paragraph 52
-URI http://asc.fasb.org/extlink&oid=109256448&loc=d3e4984-109258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e1252-109256

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e1337-109256

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109260490&loc=SL5780133-109256

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

+ Details

Common Stock - Additional Information (Detail) $ / shares in Units, $ in Millions					9 Months Ended
	Apr. 09, 2019	Sep. 11, 2018 $ / shares shares	Aug. 14, 2018 USD ($) $ / shares shares	Aug. 08, 2018 shares	Jun. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares shares
Class of Warrant or Right [Line Items]
Warrants term					5 years
Warrants of common stock exercise price \| $ / shares					$ 0.004
Maximum percentage of stock pertaining to restrictions					24.90%
Extended term of outstanding warrants expiration period					5 years
Warrants of common stock expiration date					Sep. 30, 2023
Warrants to purchase shares of common stock					250,000
Common stock, stock split					Effected a 2.5-for-1 stock split
Stock split ratio				2.5
Common stock, shares authorized				125,000,000	125,000,000	125,000,000
Preferred stock, shares authorized				5,000,000	5,000,000	5,000,000
Stock issued during period, shares, new issues		1,344,500
Common stock, no par value \| $ / shares					$ 0	$ 0
Common stock, warrants issued					4,603,333	10,614,990
IPO [Member]
Class of Warrant or Right [Line Items]
Stock issued during period, shares, new issues			9,630,000
Common stock, no par value \| $ / shares
Shares issued, price per share \| $ / shares			$ 12.00
Gross proceeds of initial public offering \| $			$ 115.6		$ 124.2
Net proceeds from initial public offering \| $					111.7
Underwriting discounts and commissions \| $					8.7
Offering costs \| $					$ 3.8
Underwriter Options [Member]
Class of Warrant or Right [Line Items]
Common stock, warrants issued			1,444,500
Purchased Directly From Company [Member]
Class of Warrant or Right [Line Items]
Stock issued during period, shares, new issues		723,985
Purchased Directly From Selling Shareholders [Member]
Class of Warrant or Right [Line Items]
Stock issued during period, shares, new issues		620,515
Firm Shares and Option Shares [Member]
Class of Warrant or Right [Line Items]
Shares issued, price per share \| $ / shares		$ 12.00
2018 Equity Incentive Plan [Member]
Class of Warrant or Right [Line Items]
Common stock, stock split					2.5-for-1 stock split effected
Stock split ratio	2.5
Increase in number of shares authorized for issuance				1,000,000

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Income Taxes [Line Items]
Effective tax rate (ETR) from continuing operations	22.20%	23.90%	23.60%	228.20%
Statutory federal tax rate	21.00%		21.00%		24.50%
State net operating loss carryforwards					$ 0.9
Domestic Tax Authority [Member] \| 2027-2037 [Member]
Income Taxes [Line Items]
Net operating loss carryforwards					$ 415.1
State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year					2019
State and Local Jurisdiction [Member] \| 2019-2038 [Member]
Income Taxes [Line Items]
Net operating loss carryforwards					$ 199.6
Minimum [Member] \| Domestic Tax Authority [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year					2027
Minimum [Member] \| State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year					2019
Maximum [Member] \| Domestic Tax Authority [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year					2037
Maximum [Member] \| State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year					2038

Share-Based Compensation and Stock Repurchases - Schedule of Restricted Share Activity (Detail) - Restricted Stock [Member]	9 Months Ended
	Jun. 30, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, Beginning Balance \| shares	1,250,625
Number of Shares, Granted \| shares	306,843
Number of Shares, Vested \| shares	(616,332)
Number of Shares, Forfeited \| shares	(22,995)
Number of Shares, Unvested, Ending Balance \| shares	918,141
Weighted-Average Grant Date Fair Value, Unvested, Beginning Balance \| $ / shares	$ 9.59
Weighted-Average Grant Date Fair Value, Granted \| $ / shares	8.89
Weighted-Average Grant Date Fair Value, Vested \| $ / shares	9.59
Weighted-Average Grant Date Fair Value, Forfeited \| $ / shares	12.00
Weighted-Average Grant Date Fair Value, Unvested, Ending Balance \| $ / shares	$ 9.29

Share-Based Compensation and Stock Repurchases - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to unvested share-based compensation arrangements	$ 7,000			$ 7,000
Unrecognized compensation cost, period for recognition				1 year 6 months
Share-based compensation expense	1,500		$ 11,600	$ 4,300	$ 12,600
Repurchased shares, value	$ 1,065	$ 449	$ 3,259
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchased shares, shares				169,724	29,710
Repurchased shares, value				$ 1,500	$ 500
Board of Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
Restricted Stock Units [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Restricted Stock Units [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Restricted Stock Award \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Restricted Stock Award \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years

Commitments - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
Commitments - Additional Information (Detail) $ in Millions	Jun. 30, 2019 USD ($) Air-craft	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Air-craft	Jun. 30, 2018 USD ($)
Operating Leased Assets [Line Items]
Number of leased aircraft \| Air-craft	18		18
Aggregate rental expense under all operating aircraft, equipment and facility leases \| $	$ 18.1	$ 21.6	$ 55.9	$ 62.8
Aircraft [Member]
Operating Leased Assets [Line Items]
Operating leases remaining term	4 years 9 months 18 days		4 years 9 months 18 days
Headquarters and other facility [Member]
Operating Leased Assets [Line Items]
Operating leases remaining term	6 years 4 months 24 days		6 years 4 months 24 days

Commitments - Schedule of Future Minimum Lease Payments (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Operating Leased Assets [Line Items]
Remainder of 2019	$ 7,363
2020	47,068
2021	45,979
2022	31,090
2023	13,726
Thereafter	14,553
Total	159,779
Aircraft [Member]
Operating Leased Assets [Line Items]
Remainder of 2019	6,265
2020	44,231
2021	44,314
2022	29,751
2023	12,418
Thereafter	11,849
Total	148,828
Other [Member]
Operating Leased Assets [Line Items]
Remainder of 2019	1,098
2020	2,837
2021	1,665
2022	1,339
2023	1,308
Thereafter	2,704
Total	$ 10,951

Supplemental Disclosure - Reconciliation of Captions in Condensed Consolidated Balance Sheets to Condensed Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 79,909	$ 103,311
Restricted cash	3,647	3,823
Cash, cash equivalents, and restricted cash in Condensed Consolidated Statement of Cash Flows	$ 83,556	$ 107,134	$ 45,554	$ 60,347

Subsequent Events - Additional Information (Detail)		9 Months Ended	12 Months Ended
Subsequent Events - Additional Information (Detail)	Aug. 12, 2019 Air-craft Extension	Jun. 30, 2019 Air-craft Extension	Sep. 30, 2018 Air-craft Extension
Subsequent Event [Line Items]
Number of aircrafts operated		145
United [Member]
Subsequent Event [Line Items]
Number of extension \| Extension		4	4
Renewal term extension period		2 years	2 years
Maximum [Member] \| United [Member]
Subsequent Event [Line Items]
Aggregate extension period		8 years	8 years
E-175 Aircraft [Member]
Subsequent Event [Line Items]
Number of aircrafts operated		60
E-175 Aircraft [Member] \| United [Member]
Subsequent Event [Line Items]
Number of aircrafts operated		30	12
E-175 Aircraft [Member] \| Minimum [Member] \| United [Member]
Subsequent Event [Line Items]
Early termination right, expiration period		2019-06
E-175 Aircraft [Member] \| Maximum [Member] \| United [Member]
Subsequent Event [Line Items]
Early termination right, expiration period		2020-08
Subsequent Event [Member] \| E-175 Aircraft [Member] \| United [Member]
Subsequent Event [Line Items]
Purchase agreement, extended period	2 years
Number of operated aircrafts expired or set to expire	7
Remaining number of aircraft operated	23
Number of aircrafts operated	30
Number of extension \| Extension	4
Renewal term extension period	2 years
Subsequent Event [Member] \| E-175 Aircraft [Member] \| Minimum [Member] \| United [Member]
Subsequent Event [Line Items]
Early termination right, expiration period	2019-10
Subsequent Event [Member] \| E-175 Aircraft [Member] \| Maximum [Member] \| United [Member]
Subsequent Event [Line Items]
Early termination right, expiration period	2020-08
Aggregate extension period	8 years

Condensed Consolidated Statements of Operations (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Contract Revenue [Member]
Revenue from related party	$ 93,683	$ 90,834	$ 282,048	$ 266,037
Pass Through and Other [Member]
Revenue from related party	$ 1,642	$ 1,634	$ 5,381	$ 4,989

Balance Sheet Information - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Depreciation expense	$ 19.3	$ 15.9	$ 56.2	$ 47.3
Amortization of unfavorable lease liability	$ 1.4	$ 1.7	$ 4.5	$ 5.1

Intangible Assets - Information About Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Intangible Assets Net Excluding Goodwill [Abstract]
Customer relationship	$ 43,800	$ 43,800
Accumulated amortization	(33,816)	(32,459)
Total	$ 9,984	$ 11,341

Long-Term Debt and Other Borrowings - Schedule of Long-term Debt (Parenthetical) (Detail)				1 Months Ended					9 Months Ended	12 Months Ended
	Jun. 14, 2019 USD ($)	Jun. 28, 2018 USD ($) Air-craft	Jun. 27, 2018 USD ($) Air-craft	Jun. 30, 2019 USD ($) Airfleet	Jan. 31, 2019 USD ($)	Jun. 30, 2018 USD ($) Air-craft	Feb. 28, 2018 Engine	Dec. 31, 2017 USD ($) Air-craft	Jun. 30, 2019 USD ($) Airfleet	Sep. 30, 2018	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) Air-craft	Sep. 30, 2015 USD ($) Air-craft	Sep. 30, 2007 USD ($) Airfleet	Sep. 30, 2005 USD ($) Airfleet	Sep. 30, 2004 USD ($) Airfleet	Sep. 30, 2014 USD ($) Airfleet
CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 69,600,000	$ 27,500,000
Long term debt interest rate description		LIBOR plus a spread ranging from 3.50% for the senior promissory notes to 7.50% for the subordinated promissory notes	LIBOR plus 3.50%.
Long term debt basis spread on variable rate			3.50%
Number of aircraft financed \| Air-craft		6	6
CRJ-700 Aircraft [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 70,000,000
Long term debt interest rate description	LIBOR plus 5.25%.
Long term debt basis spread on variable rate	5.25%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2019	2019
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Number of aircraft refinanced \| Airfleet															5	9
Debt instrument, face amount															$ 118,000,000
Long term debt interest rate description									London Inter-bank Offered Rate ("LIBOR"), plus 3.00%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate															3.00%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| CRJ-700
Debt Instrument [Line Items]
Number of aircraft refinanced \| Airfleet																5
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| Five C- R- J-700 and Nine C- R- J-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount																$ 254,700,000
Long term debt interest rate description									monthly LIBOR plus 3.00%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| Five C- R- J-700 and Nine C- R- J-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate																3.00%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2022	2022
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Number of aircraft refinanced \| Airfleet														3
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| CRJ-700
Debt Instrument [Line Items]
Number of aircraft refinanced \| Airfleet														3
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Three C- R- J-900 and Three C- R- J-700 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount														$ 120,300,000
Long term debt interest rate description									monthly LIBOR plus 2.25%
Long term debt interest rate percentage				4.638%					4.638%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Three C- R- J-900 and Three C- R- J-700 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate														2.25%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Ten C- R- J-900 Aircraft [Member]
Debt Instrument [Line Items]
Debt instrument, face amount																	$ 88,400,000
Long term debt interest rate description									LIBOR plus a spread ranging from 1.95% to 7.25%
Number of aircraft financed \| Airfleet																	10
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Ten C- R- J-900 Aircraft [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				4.338%					4.338%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Ten C- R- J-900 Aircraft [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				9.638%					9.638%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Ten C- R- J-900 Aircraft [Member] \| LIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				1.95%					1.95%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Ten C- R- J-900 Aircraft [Member] \| LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				7.25%					7.25%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2024	2024
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2024 [Member] \| Eight C- R- J-900 Aircraft [Member]
Debt Instrument [Line Items]
Debt instrument, face amount																	$ 114,500,000
Number of aircraft financed \| Airfleet																	8
Long term debt interest rate percentage																	5.00%
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2027	2027
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2027 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount													$ 170,200,000
Number of aircraft financed \| Air-craft													7
Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2028	2028
Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Ten E-175 Aircraft [Member]
Debt Instrument [Line Items]
Debt instrument, face amount												$ 246,000,000
Number of aircraft financed \| Air-craft												10
Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Ten E-175 Aircraft [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				4.75%					4.75%
Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Ten E-175 Aircraft [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				6.25%					6.25%
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2028	2028
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Eight E-175 Aircraft [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Number of aircraft refinanced \| Air-craft												8
Debt instrument, face amount												$ 195,300,000
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2022	2022
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2022	2022
Number of aircraft refinanced \| Air-craft						6		9
Debt instrument, face amount						$ 27,500,000		$ 74,900,000
Number of aircraft financed \| Air-craft						9
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| CRJ-900 [Member] \| Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 69,600,000
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2022	2022
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member] \| Flight Equipment [Member]
Debt Instrument [Line Items]
Debt instrument, face amount											$ 99,100,000
Long term debt interest rate description									monthly LIBOR plus 7.25%
Long term debt interest rate percentage				9.638%					9.638%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member] \| Flight Equipment [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate											7.25%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2020	2020
Debt instrument, face amount													$ 8,300,000
Long term debt interest rate percentage													5.163%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2020	2020
Debt instrument, face amount											$ 11,900,000	$ 11,900,000
Long term debt interest rate description									Three-month LIBOR plus a spread ranging from 2.93% to 2.96%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				5.262%					5.262%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				5.292%					5.292%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member] \| LIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate											2.93%	2.93%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member] \| LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate											2.96%	2.96%
Notes Payable to Financial Institution Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2020	2020
Notes Payable to Financial Institution Due 2020 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument, face amount												$ 10,200,000	$ 10,200,000
Long term debt interest rate description									monthly LIBOR plus 3.07%
Long term debt interest rate percentage				5.402%					5.402%
Notes Payable to Financial Institution Due 2020 [Member] \| Flight Equipment Maintenance [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage												3.07%	3.07%
Notes Payable to Financial Institution Due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2019	2019
Notes Payable to Financial Institution Due 2019 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument, face amount											$ 25,000,000
Long term debt interest rate description									Tree-month LIBOR plus 3.30%
Long term debt basis spread on variable rate											3.30%
Notes Payable to Financial Institution Due 2019 [Member] \| Flight Equipment Maintenance [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				5.632%					5.632%
Other Obligations Due to Financial Institution, Collateralized by the Underlying Equipment, Due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2023	2023
Imputed interest							9.128%
Number of spare engines leased \| Engine							2
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2024 [Member] \| Flight Equipment [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					$ 91,200,000
Long term debt interest rate description									monthly LIBOR plus 3.10%
Long term debt interest rate percentage				5.488%					5.488%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2024 [Member] \| Flight Equipment [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate					3.10%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2024 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2024	2024
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year									2023	2023
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2023 [Member] \| CRJ-700 Aircraft [Member]
Debt Instrument [Line Items]
Debt instrument, face amount				$ 70,000,000					$ 70,000,000
Long term debt interest rate description									monthly LIBOR plus 5.25%
Long term debt interest rate percentage				7.638%					7.638%
Number of aircraft financed \| Airfleet				10					10
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2023 [Member] \| CRJ-700 Aircraft [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate				5.25%
Senior Notes Due Two Thousand Twenty Seven [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount													$ 151,000,000
Long term debt interest rate description									monthly LIBOR plus 2.71%
Long term debt interest rate percentage				5.098%					5.098%
Senior Notes Due Two Thousand Twenty Seven [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate													2.71%
Subordinated Notes Due Two Thousand Twenty Seven [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Imputed interest													6.25%
Debt discount													$ 8,100,000
Senior Notes Due 2008 [Member] \| Eight E-175 Aircraft [Member]
Debt Instrument [Line Items]
Long term debt interest rate description									Three-month LIBOR plus a spread ranging from 2.20% to 2.32%
Senior Notes Due 2008 [Member] \| Eight E-175 Aircraft [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, face amount												$ 172,000,000
Long term debt interest rate percentage				4.50%					4.50%
Senior Notes Due 2008 [Member] \| Eight E-175 Aircraft [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				4.652%					4.652%
Senior Notes Due 2008 [Member] \| Eight E-175 Aircraft [Member] \| LIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate												2.20%
Senior Notes Due 2008 [Member] \| Eight E-175 Aircraft [Member] \| LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate												2.32%
Subordinated Notes Due 2008 [Member] \| Eight E-175 Aircraft [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				4.50%					4.50%
Senior Notes due Two Thousand Twenty Two [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount								$ 46,900,000
Long term debt interest rate description									three-month LIBOR plus 3.50
Long term debt interest rate percentage				5.832%					5.832%
Senior Notes due Two Thousand Twenty Two [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate								3.50%
Subordinated Notes DueTwoThousandTwentyTwo [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Long term debt interest rate description									three-month LIBOR plus 4.50%
Long term debt interest rate percentage				6.832%					6.832%
Subordinated Notes DueTwoThousandTwentyTwo [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage								4.50%
Working Capital Draw Loan, Collateralized by Certain Flight Equipment and Spare Parts [Member]
Debt Instrument [Line Items]
Debt instrument, face amount												$ 35,000,000
Long term debt interest rate description									one-month LIBOR plus 4.25%
Long term debt basis spread on variable rate												4.25%
Outstanding facility borrowings				$ 0					$ 0
Working Capital Draw Loan, Collateralized by Certain Flight Equipment and Spare Parts [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage				6.638%					6.638%
Senior Notes Due 2020 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 65,800,000
Long term debt basis spread on variable rate		3.50%				3.50%
Long term debt interest rate percentage				5.832%					5.832%
Senior Notes Due 2020 [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate description									three-month LIBOR plus 3.50%
Subordinated Notes Due 2020 [Member] \| CRJ-900 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 29,800,000
Long term debt basis spread on variable rate		7.50%				7.50%
Long term debt interest rate percentage				9.832%					9.832%
Subordinated Notes Due 2020 [Member] \| CRJ-900 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate description									three month LIBOR plus 7.50%

Earnings Per Share and Equity - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 shares
Restricted Stock [Member]
Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted loss per share	418,954
Number of Warrants [Member]
Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted loss per share	160,139