Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in OCI on derivatives (effective portion), net of tax	$ 69,000	$ (49,962,000)	$ (10,882,000)	$ 49,961,000
Gain (loss) reclassification from AOCI to income estimated net amount to be transferred	13,100,000
Operating revenues	320,406,000	276,429,000	1,265,917,000	1,126,522,000
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain from Discontinued Hedge Accounting
Derivative Instruments, Gain (Loss) [Line Items]
Operating revenues			6,700,000
Fair Value Hedging | Natural Gas | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	13,500,000		13,500,000
Fair Value Hedging | Natural Gas | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	51,800,000		51,800,000
Fair Value Hedging | Natural Gas | 2015
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	6,000,000		6,000,000
Fair Value Hedging | Crude Oil | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	4,400,000		4,400,000
Fair Value Hedging | Crude Oil | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	9,800,000		9,800,000
Fair Value Hedging | Crude Oil | 2015
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	5,800,000		5,800,000
Fair Value Hedging | Natural Gas Liquids | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	11,900,000		11,900,000
Fair Value Hedging | Natural Gas Liquids | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	0		0
Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain reclassified from accumulated OCI into income (effective portion)	8,455,000	15,998,000	29,391,000	39,012,000
Loss recognized in income on derivatives (ineffective portion and amount excluded from effectiveness testing)	(22,000)	(3,042,000)	835,000	(1,372,000)
Gain (loss) recognized in income on derivatives	(92,313,000)	(45,618,000)	(70,735,000)	33,825,000
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Current period change in fair value of interest rate swap, tax	$ 42,000	$ (30,622,000)	$ (6,669,000)	$ 30,621,000