Derivative Commodity Instruments - Derivative Instruments by Income Statement Location (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) recognized in other comprehensive income on derivatives (effective portion), net of tax of $1 and ($16,424)	$ 2,000	$ (26,798,000)
Gain (loss) reclassification from AOCI to income estimated net amount to be transferred	10,900,000
Operating revenues	561,178,000	474,016,000
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain from Discontinued Hedge Accounting
Derivative Instruments, Gain (Loss) [Line Items]
Operating revenues	2,600,000
Fair Value Hedging | Natural Gas | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	44,900,000
Fair Value Hedging | Natural Gas | 2015
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	12,000,000
Fair Value Hedging | Crude Oil | 2014
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	7,400,000
Fair Value Hedging | Crude Oil | 2015
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount	7,300,000
Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain reclassified from accumulated other comprehensive income into income (effective portion)	4,054,000	17,824,000
Loss recognized in income on derivatives (ineffective portion and amount excluded from effectiveness testing)	0	(534,000)
Gain (loss) recognized in income on derivatives	(57,446,000)	(31,501,000)
Derivative commodity instruments
Derivative Instruments, Gain (Loss) [Line Items]
Current period change in fair value of derivative, tax	$ 1,000	$ (16,424,000)