Asset Retirement Obligations - Rollforward (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Asset Retirement Obligation [Line Items]
Undiscounted estimate of future cash needs	$ 70,900,000	$ 76,200,000	$ 70,900,000	$ 76,200,000
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance			33,880,000	44,952,000
Accretion expense	703,000	2,184,000	2,111,000	5,300,000
Additions and changes in estimates			0	3,748,000
Cost of reclamation work performed			(5,389,000)	(20,244,000)
Ending balance	$ 30,602,000	$ 33,756,000	$ 30,602,000	$ 33,756,000
Minimum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate			8.00%
Maximum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate			10.00%