Equity Investments And Membership Interests In Joint Ventures (Schedule Of Equity Method Investments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended						3 Months Ended
	Mar. 31, 2012	Mar. 31, 2012 Knight Hawk [Member]	Mar. 31, 2012 DKRW [Member]	Mar. 31, 2012 DTA [Member]	Mar. 31, 2012 Tenaska [Member]	Dec. 31, 2011 Tenaska [Member]	Mar. 31, 2012 Millennium [Member]	Mar. 31, 2012 Tongue River [Member]
Schedule of Equity Method Investments [Line Items]
Balance at December 31, 2011	$ 225,605	$ 135,225	$ 19,715	$ 16,086	$ 15,266	$ 15,266	$ 26,324	$ 12,989
Advances to (distributions from)	2,361	(1,801)		925			2,562	675
Equity in comprehensive income	2,553	5,243	(879)	(1,254)			(557)
Balance at March 31, 2012	230,519	138,667	18,836	15,757	15,266	15,266	28,329	13,664
Balance at December 31, 2011	35,810		30,751		5,031	5,059
Balance at March 31, 2012	$ 38,592		$ 33,561		$ 5,031	$ 5,059