Derivative Instruments - Designated and Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivatives not designated as hedging instruments
Amount of gain (loss) recognized in operating revenues			$ 187,068	$ (125,698)
Not designated as hedging instrument
Derivatives not designated as hedging instruments
Amount of gain (loss) recognized in operating revenues	$ 46,326	$ (234,693)	187,068	(125,698)
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)	1,672	15,940	2,560	28,364
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)	$ (72)	$ (72)