Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2023 USD ($) Segment AirCraft	Sep. 30, 2022 USD ($)	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	$ 3,100,000	$ 3,300,000
Asset impairment	54,343,000	171,824,000
Impairment loss held for sale	50,600,000	62,100,000	$ 0
Impairment loss on intangible assets	54,343,000	171,824,000
Lease revenue	$ 144,700,000	$ 158,400,000	$ 170,200,000
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	$ 5,100,000	$ 3,900,000
Recognized deferred credits within contract revenue	1,700,000	900,000	$ 2,400,000
Contract assets balances	8,800,000	0
Contract cost amortization	1,000,000	0	2,000,000
Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense	3,100,000	1,900,000	400,000
Deferred heavy maintenance balance, net of accumulated amortization	8,000,000	9,700,000
Engine overhaul expense	32,400,000	23,600,000	31,400,000
Engine overhaul pass-through expense	31,900,000	21,700,000	16,800,000
Airframe check expense	23,400,000	22,100,000	51,100,000
Airframe check pass-through expense	16,900,000	3,200,000	20,500,000
Impairment loss	0	10,500,000	0
Net (loss) income	(120,116,000)	(182,678,000)	16,588,000
Net cash (used in) provided by operating activities	(24,091,000)	13,362,000	132,871,000
Working capital deficit	129,300,000
Accumulated deficit	(71,119,000)	48,997,000
Cash and cash equivalents	$ 32,940,000	57,683,000
CRJ-900 Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of aircraft classified as assets held for sale | AirCraft	14
Customer Relationships [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment loss on intangible assets	$ 3,700,000	1,900,000	0
Customer Relationships [Member] | CRJ-900 Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment loss on intangible assets	3,700,000	1,900,000	0
Property and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Asset impairment	0	109,700,000	0
Other Long-lived Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Asset impairment	0	109,700,000	$ 0
Long Lived Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Asset impairment	0
Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for obsolete expendable parts and supplies	$ 4,100,000	$ 3,800,000
Percentage leased	1.00%
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