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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018
Accounting Policies [Abstract] |  |  |
Lease revenue | $ 54.9 | $ 54.6 |
Current and non-current deferred credit balances | 14.4 |  | $ 15.4
Recognized deferred credits to revenue | 1.2 | 1.0 |
Contract assets balances | 4.1 |  | $ 4.6
Contract cost amortization | 0.5 | 0.5 |
Engine overhaul expense | 4.1 | 19.5 |
Engine overhaul pass-through expense | 1.5 | 2.3 |
Airframe check expense | 1.5 | 5.6 |
Airframe check pass-through expense | $ 0.0 | $ 3.0 |

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