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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Sep. 30, 2019
Summary Of Significant Accounting Policies [Line Items]          
Current and non-current deferred credit balances $ 9.4   $ 9.4   $ 12.1
Recognized deferred credits to revenue 0.9 $ 1.3 2.9 $ 3.8  
Contract assets balances 2.4   2.4   $ 3.9
Contract cost amortization 0.3 0.6 1.2 1.8  
Engine overhaul expense 3.2 11.4 28.3 20.8  
Engine overhaul pass-through expense 0.4 1.9 3.0 3.0  
Airframe check expense 2.9 4.4 20.6 10.6  
Airframe check pass-through expense $ 0.8 $ 0.0 $ 5.9 $ 0.0  
Aircraft [Member]          
Summary Of Significant Accounting Policies [Line Items]          
Percentage leased     12.00%