Mining Properties (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Mineral Properties, Net [Abstract]
Mining Properties	$ 614,202		$ 600,096
Accumulated Amortization	(358,864)		(347,920)
Mineral Properties, Net Book	255,338	252,176	252,176
Bogoso/Prestea
Mineral Properties, Net [Abstract]
Mining Properties	132,922		128,713
Accumulated Amortization	(64,634)		(64,972)
Mineral Properties, Net Book	68,288		63,741
Bogoso Refactory
Mineral Properties, Net [Abstract]
Mining Properties	70,867		70,865
Accumulated Amortization	(41,814)		(40,662)
Mineral Properties, Net Book	29,053		30,203
Mampon
Mineral Properties, Net [Abstract]
Mining Properties	16,095		16,095
Accumulated Amortization	0		0
Mineral Properties, Net Book	16,095		16,095
Wassa/HBB
Mineral Properties, Net [Abstract]
Mining Properties	362,135		352,241
Accumulated Amortization	(243,297)		(234,847)
Mineral Properties, Net Book	118,838		117,394
Other
Mineral Properties, Net [Abstract]
Mining Properties	32,183		32,182
Accumulated Amortization	(9,119)		(7,439)
Mineral Properties, Net Book	$ 23,064		$ 24,743