Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain recognized in OCI on derivative (effective portion), net of tax	$ 112,158,000	$ 49,449,000	$ 99,923,000	$ (47,425,000)
Loss recognized in OCI on derivative (effective portion), tax effect	68,700,000	30,300,000	61,200,000	(29,100,000)
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	50,600,000
Gain (Loss) on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net			100,000
Fair Value Hedging [Member] | Natural Gas [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	3,000,000,000		3,000,000,000
Fair Value Hedging [Member] | Natural Gas [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	5,600,000,000		5,600,000,000
Fair Value Hedging [Member] | Natural Gas [Member] | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	4,600,000,000		4,600,000,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	2,300,000		2,300,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	6,100,000		6,100,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	3,900,000		3,900,000
Fair Value Hedging [Member] | Natural Gas Liquids [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	7,600,000		7,600,000
Fair Value Hedging [Member] | Natural Gas Liquids [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	1,600,000		1,600,000
Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from accumulated OCI into income (effective portion)	21,143,000	(5,770,000)	23,013,000	1,821,000
Gain (Loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	4,336,000	(740,000)	1,669,000	(2,391,000)
Gain (loss) recongnized in income on derivative	$ 123,448,000	$ (1,000)	$ 79,443,000	$ (1,000)