Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Loss recognized in OCI on derivatives (effective portion), net of tax of ($16.4) million and ($7.5) million	$ (26,798,000)	$ (12,234,000)
Gain (loss) recognized in OCI (loss) effective portion tax	16,424,000	7,498,000
Gain (loss) reclassification from AOCI to income estimated net amount to be transferred	1,600,000
Fair Value Hedging | Natural Gas | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	4,200,000
Fair Value Hedging | Natural Gas | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	9,700,000
Fair Value Hedging | Crude Oil | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	10,500,000
Fair Value Hedging | Crude Oil | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	8,200,000
Fair Value Hedging | Crude Oil | 2015
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	700,000
Fair Value Hedging | Natural Gas Liquids | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	1,600,000
Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain reclassified from accumulated OCI into income (effective portion)	17,824,000	1,872,000
Loss recognized in income on derivatives (ineffective portion and amount excluded from effectiveness testing)	(534,000)	(2,666,000)
Loss recognized in income on derivatives	$ (31,501,000)	$ (44,005,000)