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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Summary Of Significant Accounting Policies [Line Items]        
Total operating revenues $ 171,739 $ 166,952 $ 504,063 $ 486,282
Engine overhaul expense 13,100 8,400 46,200 53,500
Engine overhaul pass-through expense 4,600 0 9,800 0
Airframe check expense 6,300 6,700 19,800 16,900
Airframe check pass-through expense 1,600 2,200 7,300 2,600
Change in accounting policy , effect of increase in SARs and Phantom Stock liability     2,400  
Contract Revenue [Member] | Rental Income [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Total operating revenues $ 53,500 $ 54,500 $ 162,000 $ 163,000