Related Party Transactions (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Rio Tinto Energy America (RTEA)
Related Party Transactions
Bond amount previously guaranteed by Rio Tinto for which replacement surety bonds have been obtained		$ 445,500,000
Rio Tinto Energy America (RTEA) | Guarantee fee
Related Party Transactions
Fee charged by Rio Tinto		683,000	1,187,000
Rio Tinto Energy America (RTEA) | Rent
Related Party Transactions
Fee charged by Rio Tinto		245,000	367,000
Rio Tinto Energy America (RTEA) | Agency services
Related Party Transactions
Fee charged by Rio Tinto	712,000	245,000
Rio Tinto Energy America (RTEA) | Export sales
Related Party Transactions
Revenue	16,203,000	8,569,000
Colowyo and uranium mining venture | Transitional support services
Related Party Transactions
Revenue			1,400,000
Venture Fuels Partnership
Related Party Transactions
Equity investment in Venture Fuels Partnership (as a percent)	50.00%
Venture Fuels Partnership | Coal sales
Related Party Transactions
Revenue	$ 20,461,000	$ 27,674,000	$ 21,434,000