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Recent Accounting Pronouncements - Additional Information (Detail)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Sep. 30, 2019
USD ($)
Air-craft
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Number of leased aircraft | Air-craft   18    
Deferred income taxes   $ 15,503 $ (17,891) $ 20,515
Income tax expense (benefit)   $ 15,706 $ (17,426) $ 20,874
Accounting Standards Update 2016-02        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Number of leased aircraft | Air-craft   18    
Number of nominal operating leased aircraft | Air-craft   42    
Accounting Standards Update 2016-02 | Minimum [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Operating lease, liability   $ 130,000    
Operating lease, right-of-use asset   120,000    
Accounting Standards Update 2016-02 | Maximum [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Operating lease, liability   170,000    
Operating lease, right-of-use asset   $ 180,000    
Accounting Standards Update 2016-09 [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Deferred income taxes $ 400      
Income tax expense (benefit) 300      
Cumulative-effect adjustment increasing retained earnings $ 700