Segment Information (Other Segment Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 US Operations Molybdenum		Dec. 31, 2012 US Operations Molybdenum		Sep. 30, 2013 Canadian Operations Molybdenum		Dec. 31, 2012 Canadian Operations Molybdenum		Sep. 30, 2013 Copper-Gold (Development)		Dec. 31, 2012 Copper-Gold (Development)		Sep. 30, 2013 Operating Segments		Dec. 31, 2012 Operating Segments		Sep. 30, 2013 Inter-segment		Dec. 31, 2012 Inter-segment
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	$ 0	$ 33.6	$ 0	$ 13.4		$ 3.2	[1]	$ 20.1	[2]	$ 2.0	[1]	$ 84.0	[2]	$ 382.1	[1]	$ 480.3	[2]	$ 387.5	[1]	$ 584.9	[2]	$ 0.2	[1]	$ 0.5	[2]
Property, plant, equipment and development	112.9	203.7	387.5	584.9		262.1		285.5		173.9		189.5		2,385.2		2,058.7		2,824.4		2,538.9		3.2		5.2
Assets	3,441.1		3,441.1		3,410.2	501.1		473.9		240.9		257.1		2,465.6		2,147.2		3,441.1		3,410.2		233.5		532.0
Liabilities	$ 2,081.3		$ 2,081.3		$ 2,008.3	$ 81.5		$ 83.3		$ 23.0		$ 19.5		$ 910.2		$ 868.5		$ 2,081.3		$ 2,008.3		$ 1,066.6		$ 1,037.0

[1] Capital expenditures for the nine months ended September 30, 2013.

[2] Capital expenditures for the nine months ended September 30, 2012.