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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Accounting Policies [Abstract]
Lease revenue	$ 54.5	$ 53.5	$ 164.5	$ 162.0
Current and non-current deferred credit balances	13.4		13.4		$ 15.4
Recognized deferred credits to revenue	1.3	1.0	3.8	3.2
Contract assets balances	4.5		4.5		$ 4.6
Contract cost amortization	0.6	0.5	1.8	1.4
Engine overhaul expense	11.4	13.1	20.7	46.2
Engine overhaul pass-through expense	1.9	4.6	3.0	9.8
Airframe check expense	4.4	6.3	10.6	19.8
Airframe check pass-through expense	$ 0.0	$ 1.6	0.0	$ 7.3
Change in accounting policy , effect of increase in SARs and Phantom Stock liability			$ 2.4

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Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer.

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Amount of operating lease income from lease payments and variable lease payments paid and payable to lessor. Includes, but is not limited to, variable lease payments not included in measurement of lease receivable.

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