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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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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

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