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Leases and Commitments - Additional Information (Detail)
$ in Thousands
 3 Months Ended
Dec. 31, 2020
USD ($)
Air-craft
Engine
Dec. 31, 2019
USD ($)
Sep. 30, 2020
USD ($)
Oct. 01, 2019
USD ($)
Operating Leased Assets [Line Items]        
Operating lease, right-of-use assets $ 114,666   $ 123,251  
Number of leased aircraft | Air-craft 18      
Aggregate rental expense under all operating aircraft, equipment and facility leases $ 13,800 $ 16,800    
Operating lease current liabilities 44,712   43,932  
Operating lease noncurrent liabilities 53,570   $ 62,531  
Operating lease payments in operating cash flows $ 9,304 $ 9,534    
CF34-8C5 Engine [Member] | Letter Agreement No. 13-1 [Member]        
Operating Leased Assets [Line Items]        
Number of new spare engines to be acquired | Engine 20      
New spare engines delivery month and year description Under the terms of the Amended and Restated Letter Agreement, the Company agreed to purchase and take delivery of the 20 new space CF34-8C5 engines commencing in May 2021, with the final spare engine to be delivered in December 2021      
New Spare engines payments commencing month and year description Payments are now due in five (5) separate tranches commencing in December 2020, and in February, April, May and June 2021      
Purchase commitment amount $ 118,900      
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