Income Taxes - Significant Portion of the Deferred Tax Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets attributable to [Abstract]
Accounts receivable	$ 2.7	$ 2.1
Inventories	2.6	2.7
Employee benefit plans	24.7	28.3
Accrued liabilities	1.7	0.6
Employee incentive plans	1.2	2.4
Tax credit carryforwards	7.9	6.9
Other	0.6	0.3
Total deferred tax assets	41.4	43.3
Deferred tax liabilities attributable to: [Abstract]
Plant and equipment	(17.0)	(17.7)
Land and mineral reserves	(0.8)	(12.7)
Joint ventures	(1.0)	(2.0)
Intangible assets	(3.8)	(2.7)
Available-for-sale securities	0	(1.2)
Other	(0.6)	(0.8)
Total deferred tax liabilities	(23.2)	(37.1)
Valuation allowances	(13.1)	(5.1)
Net deferred tax asset (liability)	$ 5.1	$ 1.1