Equity Investments and Membership Interests in Joint Ventures (Schedule of Equity Method Investments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		$ 242,215	$ 225,605	$ 177,451
Investments in affiliates				43,489
Advances to (distributions from) affiliates, net		788	7,690	(6,646)
Equity in comprehensive income (loss)		(7,400)	(8,920)	(11,311)
Impairment of equity investment		(28,947)
Balance at December 31, 2013		221,456	242,215	225,605
Knight Hawk [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		149,063	135,225	131,250
Advances to (distributions from) affiliates, net		(13,536)	(7,151)	(16,621)
Equity in comprehensive income (loss)		(17,279)	(20,989)	(20,596)
Impairment of equity investment
Balance at December 31, 2013		152,806	149,063	135,225
DKRW [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		15,515	19,715	21,961
Advances to (distributions from) affiliates, net
Equity in comprehensive income (loss)		1,832	4,200	2,246
Impairment of equity investment	(13,683)
Balance at December 31, 2013			15,515	19,715
DTA [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		15,462	16,086	14,472
Advances to (distributions from) affiliates, net		3,644	4,335	6,498
Equity in comprehensive income (loss)		4,969	4,959	4,884
Impairment of equity investment
Balance at December 31, 2013		14,137	15,462	16,086
Tenaska [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		15,264	15,266	9,768
Investments in affiliates				5,500
Advances to (distributions from) affiliates, net
Equity in comprehensive income (loss)			2	2
Impairment of equity investment		(15,264)
Balance at December 31, 2013			15,264	15,266
Millennium [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		32,214	26,324
Investments in affiliates				25,000
Advances to (distributions from) affiliates, net		6,476	8,798	3,477
Equity in comprehensive income (loss)		2,796	2,908	2,153
Impairment of equity investment
Balance at December 31, 2013		35,894	32,214	26,324
Tongue River [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012		14,697	12,989
Investments in affiliates				12,989
Advances to (distributions from) affiliates, net		4,004	1,708
Equity in comprehensive income (loss)		282
Impairment of equity investment
Balance at December 31, 2013		18,419	14,697	12,989
Other Affiliates [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Balance at December 31, 2012
Advances to (distributions from) affiliates, net		200
Equity in comprehensive income (loss)
Impairment of equity investment
Balance at December 31, 2013		$ 200