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Accounting for Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]    
Loss on derivatives, net $ (21,456) $ (9,749)
Total current period settlements on derivatives 7,414 11,272
Non-trading    
Derivative Instruments, Gain (Loss) [Line Items]    
Loss on derivatives, net (21,037) (9,620)
Total current period settlements on derivatives 7,574 11,277
Non-trading | Censtar and Oasis Power Holdings    
Derivative Instruments, Gain (Loss) [Line Items]    
Loss on derivatives, net 100 (800)
Non-trading | Provider Companies and Major Energy Companies    
Derivative Instruments, Gain (Loss) [Line Items]    
Loss on derivatives, net (1,300)  
Trading    
Derivative Instruments, Gain (Loss) [Line Items]    
Loss on derivatives, net (419) (129)
Total current period settlements on derivatives $ (160) $ (5)