Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Tax	$ (30,622,000)	$ 105,683,000	$ 30,621,000	$ 76,616,000
Gain recognized in OCI on derivative (effective portion), net of tax	(49,962,000)	172,430,000	49,961,000	125,005,000
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	20,200,000
Gain (Loss) on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net			0
Fair Value Hedging [Member] | Natural Gas [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	2,000,000		2,000,000
Fair Value Hedging [Member] | Natural Gas [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	5,600,000		5,600,000
Fair Value Hedging [Member] | Natural Gas [Member] | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	9,700,000		9,700,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	1,200,000		1,200,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	8,700,000		8,700,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	5,400,000		5,400,000
Fair Value Hedging [Member] | Natural Gas Liquids [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	3,800,000		3,800,000
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)	15,998,000	11,207,000	39,012,000	13,028,000
Gain (Loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	(3,042,000)	5,631,000	(1,372,000)	3,240,000
Derivative Instruments, Loss Recognized in Income	(45,618,000)
Derivative Instruments, Gain Recognized in Income		$ 53,227,000	$ 33,825,000	$ 53,225,000