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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