Inventory (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Inventory Disclosure [Abstract]
Finished product	$ 42.2		$ 42.2		$ 53.5
Work-in-process	29.3		29.3		32.3
Stockpiled ore	27.3		27.3		25.0
Product inventory	98.8		98.8		110.8
Inventory [Line Items]
Product inventory write-downs	13.7	29.5	27.6	57.2
Inventory write downs in operating expenses	11.8	21.6	23.4	42.5
Inventory write downs in depreciation, depletion and amortization		7.9		14.7
Operating Expense [Member]
Inventory [Line Items]
Product inventory write-downs	5.4		17.0
Depreciation, Depletion and Amortization [Member]
Inventory [Line Items]
Product inventory write-downs	1.0		3.3
Start-up Costs
Inventory [Line Items]
Product inventory write-downs	7.3		7.3
Depreciation, depletion, and amortization	$ 0.9		$ 0.9