Income Taxes - Schedule of Temporary Differences That Give Rise to Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Deferred compensation	$ 10.8	$ 11.1
Allowance for doubtful accounts	5.7	4.2
Accrued vacation and other	10.4	5.2
Inventory valuation	13.7	14.3
Tax loss carryforwards	11.5	18.3
Unrealized loss on financial derivatives	4.8	0.5
Lease liability	111.6	1.6
Total deferred tax assets	168.5	55.2
Deferred tax liabilities:
Excess tax over book depreciation and amortization	(128.7)	(159.1)
Lease right-of-use asset	(113.8)
Total deferred tax liabilities	(242.5)	(159.1)
Net deferred income tax liabilities	$ (74.0)	$ (103.9)