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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2025 USD ($) | Jun. 30, 2024 USD ($) | Jun. 30, 2025 USD ($) AirCraft | Jun. 30, 2024 USD ($) | Sep. 30, 2024 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Deferred revenue | $ 1.3 |  | $ 3.7 |  |
Lease revenue | $ 0.2 | $ 29.5 | $ 26.1 | $ 98.1 |
Operating Lease, Lease Income, Statement of Income or Comprehensive Income [Extensible Enumeration] | Revenue From Contract With Customer Excluding Assessed Tax | Revenue From Contract With Customer Excluding Assessed Tax | Revenue From Contract With Customer Excluding Assessed Tax | Revenue From Contract With Customer Excluding Assessed Tax |
Number of air craft leased from third parties | AirCraft |  |  | 0 |  |
Current and non-current deferred credit contract liability balances | $ 0.0 |  | $ 0.0 |  | $ 4.1
Recognized deferred credits within contract revenue | 0.0 | $ 0.3 | 4.1 | $ 1.5 |
Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense | 0.0 | 0.8 | 1.3 | 2.3 |
Deferred heavy maintenance balance, net of accumulated amortization | 0.0 |  | 0.0 |  | $ 6.4
Engine overhaul expense | 5.5 | 6.8 | 24.8 | 18.1 |
Engine overhaul pass-through expense | 5.5 | 6.8 | 24.7 | 18.1 |
Airframe check expense | 5.1 | 3.6 | 21.0 | 14.7 |
Airframe check pass-through expense | $ 3.8 | $ 2.1 | $ 17.8 | $ 9.1 |
E-175 Aircraft [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Number of aircraft sold | AirCraft |  |  | 18 |  |

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