Income Taxes (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets attributable to:
Accounts receivable	$ 2.1	$ 1.1
Inventories	2.7	1.7
Employee benefit plans	28.3	25.6
Accrued liabilities	0.6	0.5
Employee incentive plans	2.4	1.3
Tax credit carryforwards	6.9	7.2
Available-for-sale securities	0	2.8
Other	0.3	0.8
Total deferred tax assets	43.3	41.0
Deferred tax liabilities attributable to:
Plant and equipment	(17.7)	(17.6)
Land and mineral reserves	(12.7)	(13.5)
Joint ventures	(2.0)	(1.8)
Intangible assets	(2.7)	(1.7)
Available-for-sale securities	(1.2)	0
Other	(0.8)	(0.5)
Total deferred tax liabilities	(37.1)	(35.1)
Valuation allowances	(5.1)	(6.8)
Net deferred tax asset (liability)	$ 1.1	$ (0.9)