Leases (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended								0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Endako Mine Joint Ventrue	Jun. 30, 2013 Equipment Facility	Jun. 30, 2013 Equipment Facility	Jun. 30, 2012 Equipment Facility	Dec. 31, 2012 Equipment Facility	Jun. 30, 2013 Equipment Facility Mt. Milligan Project	Dec. 31, 2012 Equipment Facility Mt. Milligan Project	Mar. 30, 2011 Equipment Facility Mt. Milligan Project	Jun. 30, 2013 Equipment Facility Endako Mine Joint Ventrue	Dec. 31, 2012 Equipment Facility Endako Mine Joint Ventrue	Mar. 30, 2011 Minimum Equipment Facility	Mar. 30, 2011 Maximum Equipment Facility	Jun. 30, 2013 Endako Sale Leaseback [Member] Equipment Facility
Capital Lease
Maximum underwriting available under facility												$ 132.0
Term of borrowing under facility (in months)															48 months	60 months
Interest costs capitalized						1.5	2.8	0.3
Interest paid	27.5	13.2	35.4	13.4		1.2	2.1	0.2
Cash from sale of equipment					37.8
Capital lease obligations incurred	0	3.7	0	3.7	33.2
Capital lease obligations upfront payment					4.6
Equipment capital leases										26.7	29.8
Equipment facility sales-leaseback										66.5	42.3
Capital lease obligations	$ 97.2		$ 97.2			$ 93.2	$ 93.2		$ 72.1		$ 72.1		$ 4.0	$ 0			$ 4.0