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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]    
Engine overhaul expense $ 10.6 $ 4.1
Engine overhaul pass-through expense 1.9 1.5
Airframe check expense 7.3 1.5
Airframe check pass-through expense $ 1.2 $ 0.0
Aircraft [Member]    
Summary Of Significant Accounting Policies [Line Items]    
Percentage leased 12.00%