Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Mar. 31, 2024 USD ($) AirCraft	Mar. 31, 2023 USD ($) AirCraft	Sep. 30, 2023 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue	$ 7.9		$ 10.9
Lease revenue	$ 33.7	$ 38.0	68.6	$ 79.1
Operating Lease, Lease Income, Statement of Income or Comprehensive Income [Extensible Enumeration]	Revenue From Contract With Customer Excluding Assessed Tax	Revenue From Contract With Customer Excluding Assessed Tax
Number of air craft leased from third parties | AirCraft				18
Current and non-current deferred credit contract liability balances	$ 4.7		4.7		$ 5.1
Recognized deferred credits within contract revenue	0.3	$ 1.3	1.3	$ 1.5
Amortization of heavy maintenance and major overhaul costs charged to depreciation and amortization expense	0.7	0.8
Deferred heavy maintenance balance, net of accumulated amortization	6.5		6.5		$ 8.0
Engine overhaul expense	5.5	8.1	11.3	16.8
Engine overhaul pass-through expense	5.5	8.0	11.2	16.6
Airframe check expense	4.6	3.8	11.0	8.8
Airframe check pass-through expense	$ 3.2	$ 1.9	$ 7.0	$ 6.3
CRJ 700 [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of aircraft to be leased | AirCraft			2