Segment Information (Other Segment Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Mar. 31, 2012 US Operations Molybdenum		Jun. 30, 2013 US Operations Molybdenum		Mar. 31, 2012 Canadian Operations Molybdenum		Jun. 30, 2013 Canadian Operations Molybdenum		Mar. 31, 2012 Copper-Gold (Development)		Jun. 30, 2013 Copper-Gold (Development)		Mar. 31, 2012 Operating Segments		Jun. 30, 2013 Operating Segments		Mar. 31, 2012 Inter-segment		Jun. 30, 2013 Inter-segment
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	$ 14.0	$ 0.1	$ 0	$ 20.2		$ 8.8	[1]	$ 1.1	[2]	$ 71.6	[1]	$ 1.0	[2]	$ 300.4	[1]	$ 272.3	[2]	$ 381.2	[1]	$ 274.6	[2]	$ 0.4	[1]	$ 0.2	[2]
Property, plant, equipment and development	119.5	193.3	274.6	381.2		285.5		268.4		189.5		173.2		2,058.7		2,239.6		2,538.9		2,685.6		5.2		4.4
Assets	3,401.1		3,401.1		3,410.2	473.9		495.4		257.1		241.3		2,147.2		2,337.4		3,410.2		3,401.1		532.0		327.0
Liabilities	$ 2,088.0		$ 2,088.0		$ 2,008.3	$ 83.3		$ 82.3		$ 19.5		$ 21.5		$ 868.5		$ 919.7		$ 2,008.3		$ 2,088.0		$ 1,037.0		$ 1,064.5

[1] Capital expenditures for the six months ended June 30, 2012.

[2] Capital expenditures for the six months ended June 30, 2013.