Inventory (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Inventory Disclosure [Abstract]
Finished product	$ 36.1		$ 36.1		$ 53.5
Work-in-process	27.9		27.9		32.3
Stockpiled ore	25.0		25.0		25.0
Product inventory	89.0		89.0		110.8
Product inventory write-downs	8.3	16.6	13.9	27.7
Inventory write downs in operating expenses	6.9	12.6	11.6	20.9
Inventory write downs in depreciation, depletion and amortization	$ 1.4	$ 4.0	$ 2.3	$ 6.8