Equity Investments And Membership Interests In Joint Ventures (Narrative) (Details) (USD $)	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2010	Jun. 30, 2010	Jun. 30, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 31, 2010
Schedule of Equity Method Investments [Line Items]
Gain on Knight Hawk transaction						$ (41,577,000)
Investments in and Advances to Affiliates, at Fair Value, Gross Additions					43,489,000	87,405,000
Future development costs, maximum limit				72,900,000
Tongue River [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and Advances to Affiliates, at Fair Value, Gross Additions					12,989,000
Millennium [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest				38.00%
Investments in and Advances to Affiliates, at Fair Value, Gross Additions					25,000,000
Tenaska [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest				35.00%
Investments in and Advances to Affiliates, at Fair Value, Gross Additions					5,500,000	9,768,000
Future development costs, percentage				35.00%
Future development costs, maximum limit				12,500,000
DTA [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest				21.875%
Investments in and Advances to Affiliates, at Fair Value, Gross Additions
DKRW [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest				24.00%
Investments in and Advances to Affiliates, at Fair Value, Gross Additions
Interest Rate on Note Receivable				15.00%
Knight Hawk [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	49.00%	42.00%	42.00%			49.00%	33.33%
Additional ownership interest		9.00%
Gain on Knight Hawk transaction			(41,600,000)
Investments in and Advances to Affiliates, at Fair Value, Gross Additions	26,600,000					77,637,000
Reserves [Member] | Knight Hawk [Member]
Schedule of Equity Method Investments [Line Items]
Tons of Coal		68,400,000	68,400,000
Goods and Services Exchanged for Equity Instrument [Member] | Knight Hawk [Member]
Schedule of Equity Method Investments [Line Items]
Mineral Properties, Net		12,100,000	$ 12,100,000
Tongue River [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest				35.00%