Equity Investments And Membership Interests In Joint Ventures (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended				1 Months Ended		3 Months Ended	12 Months Ended					12 Months Ended				12 Months Ended				12 Months Ended				1 Months Ended	12 Months Ended				12 Months Ended
	Jun. 30, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2010 Knight Hawk [Member]	Jun. 30, 2010 Knight Hawk [Member]	Jun. 30, 2010 Knight Hawk [Member]	Dec. 31, 2011 Knight Hawk [Member]	Dec. 31, 2010 Knight Hawk [Member]	Dec. 31, 2009 Knight Hawk [Member]	May 31, 2010 Knight Hawk [Member]	Dec. 31, 2008 Knight Hawk [Member]	Dec. 31, 2011 DKRW [Member]	Dec. 31, 2010 DKRW [Member]	Dec. 31, 2009 DKRW [Member]	Dec. 31, 2008 DKRW [Member]	Dec. 31, 2011 DTA [Member]	Dec. 31, 2010 DTA [Member]	Dec. 31, 2009 DTA [Member]	Dec. 31, 2008 DTA [Member]	Dec. 31, 2011 Tenaska [Member]	Dec. 31, 2010 Tenaska [Member]	Dec. 31, 2009 Tenaska [Member]	Dec. 31, 2008 Tenaska [Member]	Jan. 31, 2011 Millennium [Member]	Dec. 31, 2011 Millennium [Member]	Dec. 31, 2010 Millennium [Member]	Dec. 31, 2009 Millennium [Member]	Dec. 31, 2008 Millennium [Member]	Dec. 31, 2011 Tongue River [Member]	Dec. 31, 2010 Tongue River [Member]	Dec. 31, 2009 Tongue River [Member]	Jul. 31, 2011 Tongue River [Member]	Dec. 31, 2008 Tongue River [Member]	Jun. 30, 2010 Reserves [Member] Knight Hawk [Member]	Jun. 30, 2010 Goods and Services Exchanged for Equity Instrument [Member] Knight Hawk [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest						49.00%	42.00%	42.00%		49.00%		33.33%		24.00%				21.875%				35.00%				38.00%								33.00%
Tons Of Coal																																				68,400,000
Additional ownership interest							9.00%	9.00%
Nonmonetary Transaction, Gain (Loss) Recognized on Transfer	$ 41,600,000		$ 41,577,000				$ 41,600,000
Mineral Properties, Net																																					12,100,000
Investments in and Advances to Affiliates, at Fair Value, Period Increase (Decrease)		(6,646,000)	(8,375,000)	(2,239,000)					(16,621,000)	(12,639,000)	(5,164,000)							6,498,000	4,264,000	2,925,000							3,477,000
Investments in and Advances to Affiliates, at Fair Value, Gross Additions		43,489,000	87,405,000			26,600,000				77,637,000												5,500,000	9,768,000			25,000,000	25,000,000				12,989,000
Interest Rate on Note Receivable														1.25%
Notes receivable from investees		35,810,000	23,900,000			1,700,000				1,700,000				30,751,000	18,100,000							5,059,000	4,100,000
Future development costs, percentage																						35.00%
Future development costs, maximum limit		74,400,000																				12,500,000
Equity investments		$ 225,605,000	$ 177,451,000	$ 87,268,000	$ 87,761,000	$ 131,250,000			$ 135,225,000	$ 131,250,000	$ 49,603,000		$ 48,093,000	$ 19,715,000	$ 21,961,000	$ 23,589,000	$ 25,124,000	$ 16,086,000	$ 14,472,000	$ 14,076,000	$ 14,544,000	$ 15,266,000	$ 9,768,000				$ 26,324,000				$ 12,989,000