Derivative Instruments - Designated and Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivatives not designated as hedging instruments
Amount of gain (loss) recognized in operating revenues			$ 222,693	$ (32,342)
Not designated as hedging instrument
Derivatives not designated as hedging instruments
Amount of gain (loss) recognized in operating revenues	$ 35,625	$ 93,356	222,693	(32,342)
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)	1,451	14,740	4,011	43,104
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)	$ (108)	$ (108)