Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Loss recognized in OCI on derivative (effective portion), net of tax of ($7.5) million and ($59.4) million	$ (12,234,000)	$ (96,873,000)
Loss recognized in OCI on derivative (effective portion), tax effect	7,500,000	59,400,000
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	200,000
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] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	840,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	3,300,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	6,100,000
Fair Value Hedging [Member] | Crude Oil [Member] | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	3,900,000
Fair Value Hedging [Member] | Natural Gas Liquids [Member] | 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	11,300,000
Fair Value Hedging [Member] | Natural Gas Liquids [Member] | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount	1,600,000
Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from accumulated OCI into income (effective portion)	1,872,000	7,591,000
Loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	(2,666,000)	(1,651,000)
Gain (loss) recongnized in income on derivative	$ (44,005,000)	$ (1,000)