Equity Method Investments and Membership Interests in Joint Ventures (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Knight Hawk [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest (percent)			49.00%
DTA [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest (percent)				21.875%
Millennium [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest (percent)				38.00%
Impairment of equity investment	$ (38,000)
Tongue River [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest (percent)				35.00%
Impairment of equity investment		$ (21,300)