Income Taxes - Components of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 229.6	$ 383.0
Alternative minimum tax credits	8.9	10.2
Inventories		1.2
Compensation	1.4	3.4
Pension benefits	7.5	6.9
Intangible assets	15.7	10.1
Other	4.5	1.7
Total deferred tax assets	267.6	416.5
Deferred tax valuation allowance	(256.1)	(406.0)	$ (382.4)	$ (385.6)
Deferred tax assets, net of valuation allowance	11.5	10.5
Deferred tax liabilities:
Property, plant and equipment	(4.0)	(11.5)
Other	(1.2)	(1.5)
Total deferred tax liabilities	(5.2)	(13.0)
Net deferred tax assets	$ 6.3
Net deferred tax liabilities		$ (2.5)