Summary of Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended							12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of quarterly results of operations
Sales of metal	$ 343,575	$ 231,090	$ 199,624	$ 207,586	$ 118,564	$ 101,018	$ 88,289	$ 246,911	$ 1,021,200	$ 515,457	$ 300,361
Income (loss) from continuing operations	31,060	38,611	12,464	(5,078)	(23,259)	(44,801)	(10,046)	29,311	93,499	(83,184)	(43,002)
Income (loss) from discontinued operations, net of income taxes	0	0	0		631	(5,943)	(2,812)			(6,029)	(9,601)
NET INCOME (LOSS)	31,060	38,611	12,464	(5,078)	(22,628)	(50,744)	(12,858)	29,311	93,499	(91,308)	(27,066)
Depreciation, depletion, and amortization	58,652	57,641	50,041	45,062	37,801	29,983	28,773	55,829	224,500	141,619	81,376
Production costs	141,253	77,102	92,474	87,625	60,402	58,590	51,019	109,127	419,956	257,636	191,311
Explorations	4,772	4,077	2,762	4,764	3,804	3,161	2,520	7,517	19,128	14,249	13,056
Other operating expenses	11,507	12,931	15,805	3,417	6,045	7,424	8,180	10,577
Cash provided by operating activities	181,911	111,065	35,786	129,399	12,939	32,456	(9,231)	87,411	416,173	165,563	60,147
Capital expenditures	$ 38,099	$ 25,764	$ 15,918	$ 26,555	$ 36,783	$ 45,467	$ 47,189	$ 40,207	$ 119,988	$ 155,994	$ 218,235
Basic income (loss) per share:
Income (loss) from continuing operations	$ 0.35	$ 0.43	$ 0.14	$ (0.06)	$ (0.26)	$ (0.51)	$ (0.12)	$ 0.13	$ 1.05	$ (0.95)	$ (0.60)
Income (loss) from discontinued operations	$ 0	$ 0	$ 0		$ 0.01	$ (0.07)	$ (0.04)			$ (0.10)	$ 0.22
Net income (loss)	$ 0.35	$ 0.43	$ 0.14	$ (0.06)	$ (0.25)	$ (0.58)	$ (0.16)	$ 0.13	$ 1.05	$ (1.05)	$ (0.38)
Diluted income (loss) per share:
Income (loss) from continuing operations	$ 0.35	$ 0.43	$ 0.14	$ (0.06)	$ (0.26)	$ (0.51)	$ (0.12)	$ 0.13	$ 1.04	$ (0.95)	$ (0.60)
Income (loss) from discontinued operations	$ 0	$ 0	$ 0		$ 0.01	$ (0.07)	$ (0.04)			$ (0.10)	$ 0.22
Net income (loss)	$ 0.35	$ 0.43	$ 0.14	$ (0.06)	$ (0.25)	$ (0.58)	$ (0.16)	$ 0.13	$ 1.04	$ (1.05)	$ (0.38)