Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2018
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 have been derived from, and 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, 2017 included in the Company’s Registration Statement on Form S-1, FileNo. 333-226173 on file with the U.S. Securities and Exchange Commission (the “SEC”). There have not been any significant changes to the Company’s significant accounting policies during the nine months ended June 30, 2018, except for the valuation of stock compensation at fair value. 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 the 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.
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.
Rental Income	Rental Income The amount deemed to be rental income for the three months ended June 30, 2018 and 2017 was $53.5 million and $54.5 million, respectively, and for the nine months ended June 30, 2018 and 2017 was $162.0 million and $163.0 million, respectively. Rental income has been included in contract revenue on the Company’s condensed consolidated statements of operations. The Company has not separately stated aircraft rental income and aircraft rental expense in the condensed consolidated statements of operations since the use of the aircraft is not a separate activity of the total service provided.
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 expense is recognized when the maintenance work is completed, or over the period of repair, 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. Engine overhaul expense totaled $13.1 million and $8.4 million for the three months ended June 30, 2018 and 2017, respectively, of which $4.6 million and $0 was pass-through expense and $46.2 million and $53.5 million for the nine months ended June 30, 2018 and 2017, respectively, of which $9.8 million and $0 was pass-through expense. Airframe check expense totaled $6.3 million and $6.7 million for the three months ended June 30, 2018 and 2017, respectively, of which $1.6 million and $2.2 million was pass-through expense and $19.8 million and $16.9 million for the nine months ended June 30, 2018 and 2017, respectively, of which $7.3 million and $2.6 million was pass-through expense. Pursuant to the United capacity purchase agreement, United reimburses the Company for heavy maintenance on certain E-175 aircraft. Those reimbursements are included in pass-through and other revenue. See Note 1: “Organization and Operations” for further information.
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 for nonpublic entities. Upon becoming a public company, as defined in ASC 718, in the third quarter of 2018, the Company was required to change its methodology for valuing the SARs and Phantom Stock. While the SARs and Phantom Stock will continue to be re-measured at each quarterly reporting date, the SARs and Phantom Stock are required to be accounted for prospectively at fair value using a fair value pricing model, such as Black-Scholes. 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. 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 condensed consolidated balance sheet was recorded with the revaluation. Any future changes in fair value will be recorded as compensation expense in the condensed consolidated statement of operations.