Derivative Instruments - Designated and Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivatives not designated as hedging instruments
Amount of gain (loss) recognized in gain (loss) on derivatives not designated as hedges				$ 47,851	$ (17,879)
Not Designated as Hedging Instrument
Derivatives not designated as hedging instruments
Amount of gain (loss) recognized in gain (loss) on derivatives not designated as hedges		$ 4,259	$ (8,525)	47,851	(17,879)
Cash Flow Hedging | Commodity Derivatives
Commodity derivatives designated as cash flow hedges
Amount of loss recognized in OCI (effective portion), net of tax		0	(13,455)	0	(52,649)
Amount of gain reclassified from accumulated OCI, net of tax, into gain on sale / exchange of assets due to forecasted transactions probable to not occur		0	16,735	0	16,735
Amount of gain (loss) reclassified from accumulated OCI, net of tax, into operating revenues (effective portion)		42,581	(10,883)	83,332	(28,146)
Amount of gain (loss) recognized in operating revenues (ineffective portion)	[1]	0	987	0	(21,273)
Interest rate derivatives designated as cash flow hedges
Amount of loss reclassified from accumulated OCI, net of tax, into interest expense (effective portion)		42,581	(10,883)	83,332	(28,146)
Cash Flow Hedging | Interest Rate Contract
Commodity derivatives designated as cash flow hedges
Amount of gain (loss) reclassified from accumulated OCI, net of tax, into operating revenues (effective portion)		(36)	(36)	(72)	(72)
Interest rate derivatives designated as cash flow hedges
Amount of loss reclassified from accumulated OCI, net of tax, into interest expense (effective portion)		$ (36)	$ (36)	$ (72)	$ (72)

[1] No amounts were excluded from effectiveness testing of cash flow hedges.