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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 15, 2021 USD ($) | Jun. 30, 2021 USD ($) Air-craft | May 31, 2021 USD ($) | Apr. 30, 2021 USD ($) | Feb. 28, 2021 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Sep. 30, 2020 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Lease incentive assets |  | $ 10.1 |  |  |  | $ 10.1 |  | $ 10.1 |  |
Lease incentive obligations |  | 8.8 |  |  |  | 8.8 |  | 8.8 |  |
Amounts deferred for supplemental rent payments |  | 0.2 |  |  |  | 0.2 |  | 0.2 |  |
Deferred revenue |  |  |  |  |  | 1.9 |  | 12.0 |  |
Current and non-current deferred credit contract liability balances |  | 5.0 |  |  |  | 5.0 |  | 5.0 |  | $ 8.5
Recognized deferred credits within contract revenue |  |  |  |  |  | 0.2 | $ 0.9 | 2.1 | $ 2.9 |
Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense |  |  |  |  |  | 0.1 | 0.1 | 0.1 | 0.1 |
Deferred heavy maintenance balance, net of accumulated amortization |  | $ 2.8 |  |  |  | 2.8 |  | 2.8 |  | $ 0.0
Engine overhaul expense |  |  |  |  |  | 4.0 | 3.2 | 25.3 | 28.3 |
Engine overhaul pass-through expense |  |  |  |  |  | 2.7 | 0.4 | 14.5 | 3.0 |
Airframe check expense |  |  |  |  |  | 14.1 | 2.9 | 38.3 | 20.6 |
Airframe check pass-through expense |  |  |  |  |  | 5.6 | 0.8 | 18.3 | 5.9 |
Payroll Support Program Extension, amount granted |  |  |  |  | $ 48.7 |  |  |  |  |
Payroll Support Program Extension, total amount granted |  |  |  |  | $ 56.0 |  |  |  |  |
Payroll Support Program Extension, additional amount received |  |  |  | $ 7.3 |  |  |  |  |  |
Third Payroll Support Program, eligible and granted amount | $ 52.2 |  |  |  |  |  |  |  |  |
Third Payroll Support Program, amount received |  |  | $ 26.1 | $ 26.1 |  |  |  |  |  |
Payroll support government funds |  |  |  |  |  | 26.1 | 43.0 | 93.4 | 43.0 |
deferred payments |  |  |  |  |  |  |  | $ 26.1 |  |
Aircraft [Member] |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage leased |  |  |  |  |  |  |  | 10.00% |  |
GoJet Agreements [Member] |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Lease revenue recognized |  |  |  |  |  | $ 3.4 |  | $ 4.0 |  |
CRJ 700 [Member] |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Number of aircraft to be leased | Air-craft |  | 12 |  |  |  |  |  |  |  |
Operating lease, term of contract |  | 9 years |  |  |  | 9 years |  | 9 years |  |
Contract Revenue [Member] |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  | $ 41.2 | $ 52.0 | $ 131.8 | $ 157.7 |

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