Derivative Instruments - Designated and Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivatives not designated as hedging instruments
Amount recognized in (loss) gain on derivatives not designated as hedges			$ (125,698)	$ 47,851
Not designated as hedging instrument
Derivatives not designated as hedging instruments
Amount recognized in (loss) gain on derivatives not designated as hedges	$ (234,693)	$ 4,259	(125,698)	47,851
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)	15,940	42,581	28,364	83,332
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)