Derivative Instruments - Designated and Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivatives not designated as hedging instruments
Amount recognized in gain (loss) on derivatives not designated as hedges			$ (32,342)	$ 209,114
Not designated as hedging instrument
Derivatives not designated as hedging instruments
Amount recognized in gain (loss) on derivatives not designated as hedges	$ 93,356	$ 161,263	(32,342)	209,114
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)	14,740	39,346	43,104	122,678
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)