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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
Sep. 30, 2019
Summary Of Significant Accounting Policies [Line Items]          
Current and non-current deferred credit balances $ 10.2   $ 10.2   $ 12.1
Recognized deferred credits to revenue 0.9 $ 1.4 2.0 $ 2.6  
Contract assets balances 2.8   2.8   $ 3.9
Contract cost amortization 0.4 0.5 0.9 1.0  
Engine overhaul expense 14.5 5.2 25.1 9.4  
Engine overhaul pass-through expense 0.7 (0.4) 2.6 1.2  
Airframe check expense 10.5 4.7 17.8 6.2  
Airframe check pass-through expense $ 3.9 $ 0.0 $ 5.1 $ 0.0  
Aircraft [Member]          
Summary Of Significant Accounting Policies [Line Items]          
Percentage leased     12.00%