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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Sep. 30, 2020
USD ($)
Segment
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Summary Of Significant Accounting Policies [Line Items]      
Number of operating segments | Segment 1    
Number of reportable segments | Segment 1    
Outstanding letters of credit to be collateralized by amounts on deposit, classified as restricted cash $ 3,400,000 $ 3,600,000  
Allowance for obsolete expendable parts and supplies   642,000 $ 200,000
Reduction of unfavorable lease liability   5,700,000 6,600,000
Write off of unfavorable lease liability   750,000 1,200,000
Lease revenue 208,900,000 219,000,000.0 217,000,000.0
Current and non-current deferred credit balances 8,500,000 12,100,000 15,400,000
Recognized deferred credits to revenue 3,700,000 5,100,000 4,400,000
Contract assets balances 2,000,000.0 3,900,000 4,600,000
Contract cost amortization 1,900,000 2,400,000 1,900,000
Engine overhaul expense 40,500,000 30,000,000.0 51,200,000
Engine overhaul pass-through expense 7,000,000.0 6,000,000.0 12,300,000
Airframe check expense 23,500,000 17,200,000 21,500,000
Airframe check pass-through expense 7,200,000 400,000 $ 7,500,000
Change in accounting policy , effect of increase in SARs and Phantom Stock liability 2,400,000    
Property and Equipment [Member]      
Summary Of Significant Accounting Policies [Line Items]      
Impairment charges 0 0  
Aircraft [Member]      
Summary Of Significant Accounting Policies [Line Items]      
Allowance for obsolete expendable parts and supplies $ 2,800,000 $ 2,400,000  
Percentage leased 12.00%    
Aircraft and Rotable Spare Parts [Member]      
Summary Of Significant Accounting Policies [Line Items]      
Property and equipment, salvage value, percentage 20.00%    
Letter of Credit [Member]      
Summary Of Significant Accounting Policies [Line Items]      
Letter of credit facility, amount $ 6,000,000.0