Equity Method Investments and Membership Interests in Joint Ventures (Schedule of Equity Method Investments) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	$ 221,456us-gaap_EquityMethodInvestments	$ 242,215us-gaap_EquityMethodInvestments	$ 225,605us-gaap_EquityMethodInvestments
Investments in affiliates			0
Advances to (distributions from) affiliates, net	4,054	788	7,690
Equity in comprehensive income (loss)	(10,332)	(7,400)	(8,920)
Impairment of equity investment		28,947
Ending Balance	235,842us-gaap_EquityMethodInvestments	221,456us-gaap_EquityMethodInvestments	242,215us-gaap_EquityMethodInvestments
Knight Hawk [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	152,806	149,063	135,225
Advances to (distributions from) affiliates, net	(12,603)	(13,536)	(7,151)
Equity in comprehensive income (loss)	(18,274)	(17,279)	(20,989)
Ending Balance	158,477	152,806	149,063
DTA [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	14,137	15,462	16,086
Advances to (distributions from) affiliates, net	3,774	3,644	4,335
Equity in comprehensive income (loss)	4,173	4,969	4,959
Ending Balance	13,738	14,137	15,462
Millennium [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	35,894	32,214	26,324
Advances to (distributions from) affiliates, net	6,742	6,476	8,798
Equity in comprehensive income (loss)	2,413	2,796	2,908
Ending Balance	40,223	35,894	32,214
Tongue River [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	18,419	14,697	12,989
Investments in affiliates			0
Advances to (distributions from) affiliates, net	2,541	4,004	1,708
Equity in comprehensive income (loss)	220	282
Ending Balance	20,740	18,419	14,697
DKRW [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance		15,515	19,715
Equity in comprehensive income (loss)		1,832	4,200
Impairment of equity investment		13,683
Ending Balance			15,515
Tenaska [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance		15,264	15,266
Investments in affiliates			0
Equity in comprehensive income (loss)			2
Impairment of equity investment		15,264
Ending Balance	0		15,264
Other Affiliates [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	200
Investments in affiliates			0
Advances to (distributions from) affiliates, net	3,600	200
Equity in comprehensive income (loss)	1,136
Ending Balance	$ 2,664	$ 200

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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Gross additions to the investment in and advance to the affiliate.

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The net of the gross additions and gross reductions to the investment in and advance to the affiliate.

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