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Summary of Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Sep. 30, 2024 USD ($) Segment | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) | Sep. 30, 2021 USD ($)
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 | $ 3,000,000 | $ 3,100,000 |  |
Asset impairment | 73,709,000 | 54,343,000 | $ 171,824,000 |
Impairment loss held for sale | $ 73,700,000 | 50,600,000 | 62,100,000 |
Revenue recognition percentage | (9.80%) |  |  |
Lease revenue | $ 123,000,000 | $ 144,700,000 | $ 158,400,000 |
Deferred revenue | $ 9,600,000 |  |  |
Operating Lease, Lease Income, Statement of Income or Comprehensive Income [Extensible Enumeration] | Contract revenue | Contract revenue | Contract revenue |
Current and non-current deferred credit balances | $ 4,100,000 | $ 5,100,000 |  |
Recognized deferred credits within contract revenue | 1,800,000 | 1,700,000 | $ 900,000 |
Contract assets balances | 6,100,000 | 8,800,000 |  |
Contract cost amortization | 2,700,000 | 1,000,000 | 0 |
Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense | 3,200,000 | 3,100,000 | 1,900,000 |
Deferred heavy maintenance balance, net of accumulated amortization | 6,400,000 | 8,000,000 |  |
Engine overhaul expense | 23,000,000 | 32,400,000 | 23,600,000 |
Engine overhaul pass-through expense | 23,500,000 | 31,900,000 | 21,700,000 |
Insurance claim reimbursement net of engine overhaul expense | 500,000 |  |  |
Airframe check expense | 22,900,000 | 23,400,000 | 22,100,000 |
Airframe check pass-through expense | 16,300,000 | 16,900,000 |  | $ 3,200,000
Impairment loss | 0 | 0 | 10,500,000 |
Net (loss) income | (91,015,000) | (120,116,000) | (182,678,000) |
Cash flow provided by operations | 34,244,000 | (24,091,000) | 13,362,000 |
Working capital deficit | 113,200,000 |  |  |
Accumulated deficit | (162,134,000) | (71,119,000) |  |
Cash and cash equivalents | 15,621,000 | 32,940,000 |  |
Property and Equipment [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Asset impairment |  |  | 109,700,000 |
Other Long-lived Assets [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Asset impairment |  |  | $ 109,700,000 |
Long Lived Assets [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Asset impairment | 0 | 0 |  |
Aircraft [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Allowance for obsolete expendable parts and supplies | $ 4,700,000 | $ 4,100,000 |  |
Percentage of short term leases | 7.00% |  |  |
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 |  |  |

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