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Summary of Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Sep. 30, 2020 USD ($) Segment | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |
Number of operating segments | Segment | 1 |  |
Number of reportable segments | Segment | 1 |  |
Outstanding letters of credit to be collateralized by amounts on deposit, classified as restricted cash | $ 3,400,000 | $ 3,600,000 |
Allowance for obsolete expendable parts and supplies |  | 642,000 | $ 200,000
Reduction of unfavorable lease liability |  | 5,700,000 | 6,600,000
Write off of unfavorable lease liability |  | 750,000 | 1,200,000
Lease revenue | 208,900,000 | 219,000,000.0 | 217,000,000.0
Current and non-current deferred credit balances | 8,500,000 | 12,100,000 | 15,400,000
Recognized deferred credits to revenue | 3,700,000 | 5,100,000 | 4,400,000
Contract assets balances | 2,000,000.0 | 3,900,000 | 4,600,000
Contract cost amortization | 1,900,000 | 2,400,000 | 1,900,000
Engine overhaul expense | 40,500,000 | 30,000,000.0 | 51,200,000
Engine overhaul pass-through expense | 7,000,000.0 | 6,000,000.0 | 12,300,000
Airframe check expense | 23,500,000 | 17,200,000 | 21,500,000
Airframe check pass-through expense | 7,200,000 | 400,000 | $ 7,500,000
Change in accounting policy , effect of increase in SARs and Phantom Stock liability | 2,400,000 |  |
Property and Equipment [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Impairment charges | 0 | 0 |
Aircraft [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for obsolete expendable parts and supplies | $ 2,800,000 | $ 2,400,000 |
Percentage leased | 12.00% |  |
Aircraft and Rotable Spare Parts [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Property and equipment, salvage value, percentage | 20.00% |  |
Letter of Credit [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Letter of credit facility, amount | $ 6,000,000.0 |  |

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