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Equity Method Investments and Membership Interests in Joint Ventures (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Investments in and Advances to Affiliates, at Fair Value, Period Increase (Decrease)		$ 4,054,000	$ 788,000	$ 7,690,000
Impairment of equity investment			28,947,000
Provision for Loan, Lease, and Other Losses	(5,200,000)
Investments in affiliates				0
Knight Hawk [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		49.00%
Investments in and Advances to Affiliates, at Fair Value, Period Increase (Decrease)		(12,603,000)	(13,536,000)	(7,151,000)
DTA [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		22.00%
Investments in and Advances to Affiliates, at Fair Value, Period Increase (Decrease)		3,774,000	3,644,000	4,335,000
Millennium [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		38.00%
Investments in and Advances to Affiliates, at Fair Value, Period Increase (Decrease)		6,742,000	6,476,000	8,798,000
Tongue River [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		35.00%
Investments in and Advances to Affiliates, at Fair Value, Period Increase (Decrease)		2,541,000	4,004,000	1,708,000
Investments in affiliates				0
DKRW [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		24.00%
Other than Temporary Impairment Losses, Investments		(57,700,000)
Impairment of equity investment			13,683,000
Provision for Loan, Lease, and Other Losses	(44,000,000)
Tenaska [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		35.00%
Other than Temporary Impairment Losses, Investments		(20,500,000)
Impairment of equity investment			15,264,000
Investments in affiliates				0
Equity Method Investee [Member]
Schedule of Equity Method Investments [Line Items]
Future development costs, maximum limit		$ 58,500,000