Asset Retirement Obligations - Rollforward (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Asset Retirement Obligation [Line Items]
Undiscounted estimate of future cash needs	$ 71,900,000	$ 76,200,000	$ 71,900,000	$ 76,200,000
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance			33,880,000	44,952,000
Accretion expense	705,000	2,183,000	1,408,000	3,116,000
Additions and changes in estimates			0	3,748,000
Cost of reclamation work performed			(4,422,000)	(11,828,000)
Balance at June 30	$ 30,866,000	$ 39,988,000	$ 30,866,000	$ 39,988,000
Minimum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate			8.00%
Maximum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate			10.00%