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Accounting for Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]        
Current period settlements on derivatives     $ 8,547 $ (10,256)
Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) on derivatives, net $ (4,055) $ (4,019) (5,360) 1,440
Current period settlements on derivatives 4,357 (59) 8,547 (10,256)
Not Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) on derivatives, net (4,041) (4,438) (5,241) 7,010
Current period settlements on derivatives 4,328 (97) 8,442 (12,998)
Not Designated as Hedging Instrument | Affiliated Entity        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) on derivatives, net 0 157    
Designated as Hedging Instrument | Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) on derivatives, net (14) 419 (119) (5,570)
Current period settlements on derivatives 29 38 $ 105 2,742
Designated as Hedging Instrument | Affiliated Entity | Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) on derivatives, net 0 866   1,792
Current period settlements on derivatives $ 0 $ 866   $ 1,693