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Accounting for Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net $ 22,407 $ (18,497) $ (14,535)
Current period settlements on derivatives (2,146) 20,547 (3,479)
Non-trading      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net 22,254 (18,423) (8,713)
Current period settlements on derivatives (2,284) 20,279 (6,289)
Non-trading | Censtar and Oasis      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net 1,000 3,400  
Non-trading | Provider Companies and Major Energy Companies      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net 25,600    
Trading | Affiliated Entity      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net 0 203 1,509
Current period settlements on derivatives 0 315 (1,780)
Cash flow hedges | Non-trading | Affiliated Entity      
Derivative Instruments, Gain (Loss) [Line Items]      
Ineffectiveness gain (loss)     (288)
Non-cash Flow hedges | Non-trading      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net 22,254 (18,423) (8,713)
Current period settlements on derivatives (2,284) 20,279 (6,289)
Non-cash Flow hedges | Trading      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) on derivatives, net 153 (74) (5,822)
Current period settlements on derivatives $ 138 $ 268 $ 2,810