Summary of Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of quarterly results of operations
Sales of metal	$ 168,847	$ 200,825	$ 204,525	$ 171,797	$ 205,929	$ 230,593	$ 254,406	$ 204,564	$ 745,994	$ 895,492	$ 1,021,200
NET INCOME (LOSS)	(581,528)	(46,265)	(35,040)	12,270	37,550	(15,821)	22,973	3,975	(650,563)	48,677	93,499
Amortization	63,916	60,874	57,653	50,436	52,397	52,844	61,024	52,592	232,879	218,857	224,500
Production costs	100,227	131,728	142,924	88,784	106,473	124,365	131,182	92,542	463,663	454,562	419,547
Exploration	5,440	3,305	6,774	6,841	6,441	6,957	6,305	6,567	22,360	26,270	19,128
Other operating expenses	789,094	20,195	48,131	14,831	5,883	11,836	13,680	8,664
Cash provided by operating activities	10,391	26,804	63,338	12,934	61,694	79,735	113,203	17,002	113,467	271,634	416,173
Capital expenditures	$ 28,059	$ 32,726	$ 27,201	$ 12,827	$ 21,784	$ 29,972	$ 32,238	$ 31,647	$ 100,813	$ 115,641	$ 119,988
Basic income per share:
Net income (loss)	$ (5.77)	$ (0.46)	$ (0.35)	$ 0.14	$ 0.42	$ (0.18)	$ 0.26	$ 0.04	$ (6.65)	$ 0.54	$ 1.05
Diluted income per share:
Net income (loss)	$ (5.77)	$ (0.46)	$ (0.35)	$ 0.14	$ 0.42	$ (0.18)	$ 0.26	$ 0.04	$ (6.65)	$ 0.54	$ 1.04