Derivative Losses (Details) In Thousands, except Share data in Millions, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended								12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Mar. 31, 2008 Riverstone Resources, Inc. - warrants CAD	Dec. 31, 2012 Riverstone Resources, Inc. - warrants USD ($)	Dec. 31, 2011 Riverstone Resources, Inc. - warrants USD ($)	Dec. 31, 2010 Riverstone Resources, Inc. - warrants USD ($)	Dec. 31, 2012 Gold price derivatives USD ($)	Dec. 31, 2011 Gold price derivatives USD ($)	Dec. 31, 2010 Gold price derivatives USD ($)	Feb. 28, 2011 Gold price derivatives oz	Jan. 31, 2011 Gold price derivatives oz	Dec. 31, 2012 Riverstone Resources, Inc. - warrants and Gold Price derivatives USD ($)	Dec. 31, 2011 Riverstone Resources, Inc. - warrants and Gold Price derivatives USD ($)	Dec. 31, 2010 Riverstone Resources, Inc. - warrants and Gold Price derivatives USD ($)
Derivative [Line Items]
Derivative loss	$ 162	$ 19,276	$ 850		$ 162	$ (177)	$ (216)	$ 0	$ 19,453	$ 1,066
Realized (gain) / loss													162	19,453	1,066
Unrealized loss	$ (162)	$ 177	$ 215										$ 0	$ (177)	$ (216)
Warrants issued				2.0
Warrants exercise price (CDN$ per share)				0.45
Notional amount of derivative (ounces)											75,200	76,800
Derivative floor price (US$ per ounce)											1,200	1,200
Derivative cap price (US$ per ounce)											1,503	1,457