Inventory (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Inventory Disclosure [Abstract]
Finished product	$ 52.0		$ 53.5
Work-in-process	26.9		32.3
Stockpiled ore	26.8		25.0
Product inventory	105.7		110.8
Product inventory write-downs	5.6	11.1
Inventory write downs in operating expenses	4.7	8.3
Inventory write downs in depreciation, depletion and amortization	0.9
Prior period reclassification adjustment-inventory write-downs		$ 2.8