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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Sep. 30, 2018
Accounting Policies [Abstract] |  |  |  |  |
Lease revenue | $ 55.0 | $ 54.0 | $ 109.9 | $ 108.5 |
Current and non-current deferred credit balances | 14.7 |  | 14.7 |  | $ 15.4
Recognized deferred credits to revenue | 1.4 | 1.2 | 2.6 | 2.2 |
Contract assets balances | 5.0 |  | 5.0 |  | $ 4.6
Contract cost amortization | 0.6 | 0.5 | 1.2 | 1.0 |
Engine overhaul expense | 5.2 | 13.7 | 9.4 | 33.1 |
Engine overhaul pass-through expense | (0.4) | 2.9 | 1.2 | 5.2 |
Airframe check expense | 4.7 | 7.9 | 6.2 | 13.5 |
Airframe check pass-through expense | $ 0.0 | $ 2.7 | $ 0.0 | $ 5.7 |

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