Asset Retirement Obligations (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in carrying amount of asset retirement obligations
Balance at the beginning of the period		$ 199,633,000	$ 188,757,000
Accretion expense		13,189,000	12,469,000	12,499,000
Revisions to estimated cash flows		33,444,000	5,913,000
Payments		(5,632,000)	(7,506,000)	(5,938,000)
Balance at the end of the period		240,634,000	199,633,000	188,757,000
Less current portion		(1,643,000)	(6,926,000)
Asset retirement obligation, net of current portion		238,991,000	192,707,000
Concurrent reclamation excluded from asset retirement obligation		8,600,000	5,300,000
Additional expected life of Antelope mine		12 years
Depreciation and depletion expense	$ 15,700,000