Income Taxes - Components of Net Deferred Tax Asset (Liability) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Tax effect of temporary differences related to:
Inventories	$ 1.9	$ 3.7
Lease assets	6.3	5.9
Accrued OPEB costs	3.0	2.8
Accrued pension costs	100.5	80.6
Accrued environmental liabilities	31.0	33.8
Other deductible differences	9.2	8.3
Investments in subsidiaries and affiliates	2.7	2.6
Tax loss and tax credit carryforwards	100.4	91.2
Valuation allowance	(17.5)	(14.2)
Adjusted gross deferred tax assets	237.5	214.7
Netting of items by tax jurisdiction, assets	(117.3)	(108.7)
Net noncurrent deferred tax asset (liability)	120.2	106.0
Tax effect of temporary differences related to:
Property and equipment	(67.2)	(61.8)
Lease liabilities	(6.5)	(6.1)
Other taxable differences	(13.1)	(13.7)
Investments in subsidiaries and affiliates	(48.1)	(54.4)
Tax on unremitted earnings of non-U.S. subsidiaries	(12.0)	(10.8)
Adjusted gross deferred tax liabilities	(146.9)	(146.8)
Netting of items by tax jurisdiction, liabilities	117.3	108.7
Net noncurrent deferred tax asset (liability)	$ (29.6)	$ (38.1)