Asset Retirement Obligations - Rollforward (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Asset Retirement Obligation [Line Items]
Undiscounted estimate of future cash needs	$ 72,500,000		$ 73,400,000
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	34,113,000	33,880,000
Accretion expense	634,000	703,000
Additions and changes in estimates	0	0
Cost of reclamation work performed	(883,000)	(2,575,000)
Balance at the end of the period	$ 33,864,000	$ 32,008,000
Minimum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate	8.00%
Maximum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate	10.00%