Accounting for Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivatives, net	$ (609)	$ 61	$ 2,887	$ (6,118)
Total current period settlements on derivatives	(8,869)	4,163	3,427	12,887
Non-trading
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivatives, net	(1,183)	132	2,519	(5,876)
Total current period settlements on derivatives	(8,889)	4,035	3,341	12,643
Non-trading | Censtar and Oasis Power Holdings
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivatives, net	500	2,000	600	2,200
Non-trading | Provider Companies and Major Energy Companies
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivatives, net	11,200		14,700
Trading
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivatives, net	574	(71)	368	(242)
Total current period settlements on derivatives	$ 20	$ 128	$ 86	$ 244