Accounting for Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Current period settlements on derivatives			$ 15,120	[1]	$ (7,252)
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	$ 61	$ (1,178)	(6,118)		262
Current period settlements on derivatives	4,163	3,004	12,887		(7,252)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	132	(1,163)	(5,876)		5,847
Current period settlements on derivatives	4,035	3,039	12,643		(9,959)
Not Designated as Hedging Instrument | Censtar and Oasis Power Holdings
Derivative Instruments, Gain (Loss) [Line Items]
Current period settlements on derivatives	2,000		2,200
Not Designated as Hedging Instrument | Affiliated Entity
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	0	0
Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	(71)	(15)	(242)		(5,585)
Current period settlements on derivatives	128	(35)	244		2,707
Designated as Hedging Instrument | Affiliated Entity | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	0	0	0		1,792
Current period settlements on derivatives	$ 0	$ 0	$ 0		$ 1,693

[1] Financial information has been recast to include results attributable to the acquisition of Oasis Power Holdings LLC on May 12, 2015 from an affiliate. See Note 2 “Basis of Presentation” and Note 3 "Acquisitions" for further discussion.