Asset Retirement Obligations - Rollforward (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation [Line Items]
Undiscounted estimate of future cash needs	$ 73,400,000	$ 72,800,000
Changes in the carrying amounts of the ARO [Roll Forward]
Beginning balance	33,880,000	44,952,000
Accretion expense	2,816,000	3,845,000	2,803,000
Additions and changes in estimates	3,620,000	11,978,000
Cost of reclamation work performed	(6,203,000)	(26,895,000)
Ending balance	$ 34,113,000	$ 33,880,000	$ 44,952,000
Minimum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate	8.00%
Maximum
Asset Retirement Obligation [Line Items]
ARO Valuation discount rate	10.00%