Asset Retirement Obligations (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2011 Y	Dec. 31, 2010	Dec. 31, 2009
Changes in carrying amount of asset retirement obligations
Balance at the beginning of the period		$ 188,757,000	$ 181,419,000
Accretion expense		12,469,000	12,499,000	12,587,000
Revisions to estimated cash flows		5,913,000	777,000
Payments		(7,506,000)	(5,938,000)	(4,855,000)
Balance at the end of the period		199,633,000	188,757,000	181,419,000
Less current portion		(6,926,000)	(6,587,000)
Asset retirement obligation, net of current portion		192,707,000	182,170,000
Concurrent reclamation excluded from asset retirement obligation		5,300,000	5,900,000
Additional expected life of Antelope mine (in years)		12.0
Depreciation and depletion expense	$ 15,700,000