Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain recognized in OCI on derivatives (effective portion), net of tax of $9.7 million and $68.7 million	$ 15,847,000	$ 112,158,000	$ (10,951,000)	$ 99,923,000
Gain (loss) reclassification from AOCI to income estimated net amount to be transferred	14,600,000
Operating revenues	490,057,000	470,355,000	982,736,000	888,799,000
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain from Discontinued Hedge Accounting
Derivative Instruments, Gain (Loss) [Line Items]
Operating revenues			5,900,000
Fair Value Hedging | Natural Gas | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	27,300,000		27,300,000
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	8,500,000		8,500,000
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	700,000		700,000
Fair Value Hedging | Natural Gas Liquids | 2013
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	23,200,000		23,200,000
Fair Value Hedging | Natural Gas Liquids | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	1,900,000		1,900,000
Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain reclassified from accumulated OCI into income (effective portion)	3,112,000	21,143,000	20,935,000	23,013,000
Gain recognized in income on derivatives (ineffective portion and amount excluded from effectiveness testing)	1,392,000	4,336,000	858,000	1,669,000
Gain recognized in income on derivatives	53,078,000	123,448,000	21,577,000	79,443,000
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Current period change in fair value of interest rate swap, tax	$ 9,713,000	$ 68,741,000	$ (6,712,000)	$ 61,243,000