Derivative Instruments in the Balance Sheets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Commodity Contracts
Assets
Gross Recognized Derivatives	$ 49,449	$ 53,372
Amounts Offset	(17,973)	(22,317)
Net Recognized Derivatives	31,476	31,055
Other	626	350
Amount Reported on Balance Sheet	32,102	31,405
Liabilities
Gross Recognized Derivatives	(203,980)	(169,052)
Amounts Offset	74,873	66,217
Net Recognized Derivatives	(129,107)	(102,835)
Other	(7,023)	(7,443)
Amount Reported on Balance Sheet	(136,130)	(110,278)
Assets and Liabilities
Gross Recognized Derivatives	(154,531)	(115,680)
Amounts Offset	56,900	43,900
Net Recognized Derivatives	(97,631)	(71,780)
Other	(6,397)	(7,093)
Amount Reported on Balance Sheet	(104,028)	(78,873)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	26,414	28,562
Amounts Offset	(13,382)	(15,127)
Net Recognized Derivatives	13,032	13,435
Other	626	350
Amount Reported on Balance Sheet	13,658	13,785
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	23,035	24,810
Amounts Offset	(4,591)	(7,190)
Net Recognized Derivatives	18,444	17,620
Other	0	0
Amount Reported on Balance Sheet	18,444	17,620
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(97,349)	(86,062)
Amounts Offset	42,069	33,829
Net Recognized Derivatives	(55,280)	(52,233)
Other	(7,023)	(7,443)
Amount Reported on Balance Sheet	(62,303)	(59,676)
Assets and Liabilities
Amounts Offset	56,900	43,900
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(106,631)	(82,990)
Amounts Offset	32,804	32,388
Net Recognized Derivatives	(73,827)	(50,602)
Other	0	0
Amount Reported on Balance Sheet	(73,827)	(50,602)
Designated as Hedging Instruments
Liabilities
Gross Recognized Derivatives	$ (4,000)	$ (4,000)