Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Number of reportable segments | Segment	1
Marketable securities	$ 0	$ 19,921,000
Outstanding letters of credit to be collateralized by amounts on deposit, classified as restricted cash	3,600,000	3,800,000
Allowance for obsolete expendable parts and supplies	642,000	200,000	$ 419,000
Reduction of unfavorable lease liability	5,700,000	6,600,000	6,800,000
Write off of unfavorable lease liability	750,000	1,200,000
Lease revenue	219,000,000	217,000,000	217,600,000
Current and non-current deferred credit balances	12,100,000	15,400,000	17,200,000
Recognized deferred credits to revenue	5,100,000	4,400,000	3,800,000
Contract assets balances	3,900,000	4,600,000	6,100,000
Contract cost amortization	2,400,000	1,900,000	1,600,000
Engine overhaul expense	30,000,000	51,200,000	64,000,000
Engine overhaul pass-through expense	6,000,000	12,300,000	300,000
Airframe check expense	17,200,000	21,500,000	22,600,000
Airframe check pass-through expense	400,000	7,500,000	$ 4,900,000
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	$ 2,400,000	$ 1,800,000
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