Gold Stream Arrangement (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			1 Months Ended	9 Months Ended	12 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Aug. 31, 2012 Gold Stream Amended and Restated Agreements Mt. Milligan oz	Sep. 30, 2013 Gold Stream Amended and Restated Agreements Mt. Milligan	Dec. 31, 2012 Gold Stream Amended and Restated Agreements Mt. Milligan	Dec. 31, 2011 Gold Stream Amended and Restated Agreements Mt. Milligan	Dec. 31, 2010 Gold Stream Amended and Restated Agreements Mt. Milligan	Sep. 30, 2013 Payment, March 4, 2013 Gold Stream Amended and Restated Agreements Mt. Milligan	Sep. 30, 2013 Payment, June 1, 2013 Gold Stream Amended and Restated Agreements Mt. Milligan	Sep. 30, 2013 Payment, September 1, 2013 Gold Stream Amended and Restated Agreements Mt. Milligan
Arrangement disclosures
Percentage of payable gold to be sold per arrangement (percent)						52.25%
Total receipts on gold purchased						$ 781.5
Specified purchase price per ounce (usd per ounce)						435
Gold Stream deferred revenue	758.8		758.8		669.6		781.5
Scheduled quarterly receipts on gold purchased													12.9
Proceeds from Gold	12.9	120.0	111.9	210.0				305.0	138.1	226.5	62.0	37.0
Debt amendment, restriction against collateral						350
Refined gold sold						425,000
Gold payments in excess						$ 280
Period for which cash deposit can be used for price differential (in years)							50 years