Segment Information (Other Segment Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	$ 0		$ 20.3
Property, plant, equipment and development	155.1		187.9
Assets	3,428.0				3,410.2
Liabilities	2,049.5				2,008.3
US Operations Molybdenum
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	0.3	[1]	4.9	[2]
Property, plant, equipment and development	275.9		291.5
Goodwill			47.0
Assets	480.1		583.3
Liabilities	81.8		92.1
Canadian Operations Molybdenum
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	0.3	[1]	42.1	[2]
Property, plant, equipment and development	181.6		740.0
Goodwill			0
Assets	247.4		815.0
Liabilities	19.1		128.9
Copper-Gold (Development)
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	154.4	[1]	140.8	[2]
Property, plant, equipment and development	2,160.2		1,552.7
Goodwill			0
Assets	2,240.9		1,605.3
Liabilities	882.8		698.7
Inter-segment
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	0.1	[1]	0.1	[2]
Property, plant, equipment and development	4.9		6.2
Goodwill			0
Assets	459.6		79.5
Liabilities	1,065.8		403.5
Total
Other segment information regarding capital expenditures, assets and liabilities
Capital expenditures	155.1	[1]	187.9	[2]
Property, plant, equipment and development	2,622.6		2,590.4
Goodwill			47.0
Assets	3,428.0		3,083.1
Liabilities	$ 2,049.5		$ 1,323.2

[1] Capital expenditures are for the three months ended March 31, 2013.

[2] Capital expenditures are for the three months ended March 31, 2012.