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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Sep. 30, 2019
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Current and non-current deferred credit balances | $ 9.4 |  | $ 9.4 |  | $ 12.1
Recognized deferred credits to revenue | 0.9 | $ 1.3 | 2.9 | $ 3.8 |
Contract assets balances | 2.4 |  | 2.4 |  | $ 3.9
Contract cost amortization | 0.3 | 0.6 | 1.2 | 1.8 |
Engine overhaul expense | 3.2 | 11.4 | 28.3 | 20.8 |
Engine overhaul pass-through expense | 0.4 | 1.9 | 3.0 | 3.0 |
Airframe check expense | 2.9 | 4.4 | 20.6 | 10.6 |
Airframe check pass-through expense | $ 0.8 | $ 0.0 | $ 5.9 | $ 0.0 |
Aircraft [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Percentage leased |  |  | 12.00% |  |

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