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Leases and Commitments - Additional Information (Detail)
$ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2020
USD ($)
Air-craft
Mar. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
Air-craft
Engine
Mar. 31, 2019
USD ($)
Oct. 01, 2019
USD ($)
Operating Leased Assets [Line Items]          
Operating lease, right-of-use assets $ 140,753   $ 140,753    
Number of leased aircraft | Air-craft 18   18    
Aggregate rental expense under all operating aircraft, equipment and facility leases $ 18,600 $ 18,700 $ 35,400 $ 37,800  
Operating lease current liabilities 43,648   43,648    
Operating lease noncurrent liabilities 78,698   78,698    
Operating lease payments in operating cash flows     26,240    
CF34-8C5 Engine [Member] | Letter Agreement [Member]          
Operating Leased Assets [Line Items]          
Purchase commitment amount $ 118,900   $ 118,900    
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 Letter Agreement No. 13-1, the Company has agreed to purchase and take delivery of 20 new spare CF34-8C5 engines. Delivery of the new spare engines will now commence in April 2021, with the final spare engine being delivered in December 2021.    
New Spare engines payments commencing month and year description     The payment terms for the new spare engines were also amended, with payments to now be made in four (4) separate tranches commencing in October and December 2020 and February and March 2021.    
CF34-8C5 Engine [Member] | Letter Agreement No. 12-1 [Member]          
Operating Leased Assets [Line Items]          
Number of new spare engines to be acquired | Engine     2    
New spare engines delivery month and year description     Under Letter Agreement No. 12-1, the Company has agreed to purchase and take delivery of two (2) new spare CF34-8C5 engines by no later than March 31, 2020.    
New Spare engines payments commencing month and year description     The parties agreed that progress payments previously made by the Company constitute full payment for these engines.    
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