Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Commodity Contracts
Assets
Gross Recognized Derivatives	$ 46,045	$ 53,372
Amounts Offset	(17,712)	(22,317)
Net Recognized Derivatives	28,333	31,055
Other	4,902	350
Amount Reported on Balance Sheet	33,235	31,405
Liabilities
Gross Recognized Derivatives	(175,678)	(169,052)
Amounts Offset	35,412	66,217
Net Recognized Derivatives	(140,266)	(102,835)
Other	(8,096)	(7,443)
Amount Reported on Balance Sheet	(148,362)	(110,278)
Assets and Liabilities
Gross Recognized Derivatives	(129,633)	(115,680)
Amounts Offset	17,700	43,900
Net Recognized Derivatives	(111,933)	(71,780)
Other	(3,194)	(7,093)
Amount Reported on Balance Sheet	(115,127)	(78,873)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	25,485	28,562
Amounts Offset	(12,925)	(15,127)
Net Recognized Derivatives	12,560	13,435
Other	2,162	350
Amount Reported on Balance Sheet	14,722	13,785
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	20,560	24,810
Amounts Offset	(4,787)	(7,190)
Net Recognized Derivatives	15,773	17,620
Other	2,740	0
Amount Reported on Balance Sheet	18,513	17,620
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(83,203)	(86,062)
Amounts Offset	30,626	33,829
Net Recognized Derivatives	(52,577)	(52,233)
Other	(8,096)	(7,443)
Amount Reported on Balance Sheet	(60,673)	(59,676)
Assets and Liabilities
Amounts Offset		43,900
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(92,475)	(82,990)
Amounts Offset	4,786	32,388
Net Recognized Derivatives	(87,689)	(50,602)
Other	0	0
Amount Reported on Balance Sheet	(87,689)	(50,602)
Designated as Hedging Instruments
Liabilities
Gross Recognized Derivatives	$ (4,000)	$ (4,000)