Leases (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended				9 Months Ended
	Mar. 30, 2011	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Capital Lease
Interest paid		$ 19.1	[1]	$ 1.4	[1]	$ 54.5	[1]	$ 14.8	[1]
Capital lease obligations incurred		0	[2]	6.6	[2]	0	[2]	10.3	[2]
Mt. Milligan Project
Capital Lease
Cash from sale of equipment						37.8
Capital lease obligations incurred						33.2
Capital lease obligations upfront payment						4.6
Endako Mine Joint Ventrue
Capital Lease
Cash from sale of equipment		5.3
Capital lease obligations upfront payment		1.4
Equipment Facility
Capital Lease
Interest rate minimum	5.30%
Interest rate maximum	5.60%
Interest costs capitalized		0.9		0.5		3.7		0.8
Interest paid		1.4		0.1		3.5		0.4
Stated Interest rate (percent)		5.50%				5.50%
Expense of interest and debt issuance costs		0.5				0.5
Capital lease obligations		92.2				92.2				72.1
Basis spread on variable rate	3.45%
Equipment Facility | Mt. Milligan Project
Capital Lease
Maximum underwriting available under facility	132.0
Equipment capital leases		25.3				25.3				29.8
Equipment facility sales-leaseback		63.1				63.1				42.3
Capital lease obligations		88.4				88.4				72.1
Equipment Facility | Endako Mine Joint Ventrue
Capital Lease
Interest paid		0.1				0.1
Stated Interest rate (percent)		5.85%				5.85%
Capital lease obligations		$ 3.8				$ 3.8				$ 0
Minimum | Equipment Facility
Capital Lease
Term of borrowing under facility (in months)	48 months
Maximum | Equipment Facility
Capital Lease
Term of borrowing under facility (in months)	60 months

[1] For the three and nine months ended September 30, 2013, the cash interest paid of $19.1 million and $54.5 million, respectively, had been previously capitalized related to the Company's debt, as described in Note 9. For the three and nine months ended September 30, 2012, the cash interest paid of $1.4 million and $14.8 million, respectively, had been previously capitalized related to the Company's debt, as described in Note 9.

[2] Excluded sale-leaseback capital leases.