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Leases and Commitments - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Sep. 30, 2020
USD ($)
Air-craft
Sep. 30, 2019
USD ($)
Oct. 01, 2019
USD ($)
Operating Leased Assets [Line Items]      
Operating lease, right-of-use assets $ 123,251    
Number of leased aircraft | Air-craft 18    
Aggregate rental expense under all operating aircraft, equipment and facility leases $ 64,700 $ 72,800  
Operating lease current liabilities 43,932    
Operating lease noncurrent liabilities 62,531    
Operating lease payments in operating cash flows $ 44,173    
Accounting Standards Update 2016-02 [Member]      
Operating Leased Assets [Line Items]      
Operating lease, right-of-use assets     $ 154,600
Operating lease, liability     141,900
Accounting Standards Update 2016-02 [Member] | Prepaid Aircraft Rent [Member]      
Operating Leased Assets [Line Items]      
Operating lease, right-of-use assets     35,800
Accounting Standards Update 2016-02 [Member] | Deferred Rent Credits [Member]      
Operating Leased Assets [Line Items]      
Operating lease, right-of-use assets     21,300
Accounting Standards Update 2016-02 [Member] | Accrued Aircraft Rents [Member]      
Operating Leased Assets [Line Items]      
Operating lease, right-of-use assets     $ 1,800