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Derivative Instruments - Designated and Not Designated as Hedging Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Derivatives not designated as hedging instruments    
Amount of gain recognized in operating revenues $ 140,742 $ 108,995
Not designated as hedging instrument    
Derivatives not designated as hedging instruments    
Amount of gain recognized in operating revenues 140,742 108,995
Cash flow hedging | Designated as hedging instrument | Commodity derivatives    
Commodity/Interest rate derivatives designated as cash flow hedges    
Amount of gain (loss) reclassified from accumulated OCI, net of tax (effective portion) 888 12,424
Cash flow hedging | Designated as hedging instrument | Interest rate derivatives    
Commodity/Interest rate derivatives designated as cash flow hedges    
Amount of gain (loss) reclassified from accumulated OCI, net of tax (effective portion) $ (36) $ (36)