Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets
Gross Recognized Derivatives	$ 50,547	$ 53,372
Amounts Offset	(23,225)	(22,317)
Net Recognized Derivatives	27,322	31,055
Other	1,640	350
Amount Reported on Balance Sheet	28,962	31,405
Liabilities
Gross Recognized Derivatives	(200,001)	(169,052)
Amounts Offset	37,125	66,217
Net Recognized Derivatives	(162,876)	(102,835)
Other	(8,121)	(7,443)
Amount Reported on Balance Sheet	(170,997)	(110,278)
Assets and Liabilities
Gross Recognized Derivatives	(149,454)	(115,680)
Amounts Offset	13,900	43,900
Net Recognized Derivatives	(135,554)	(71,780)
Other	(6,481)	(7,093)
Amount Reported on Balance Sheet	(142,035)	(78,873)
Current Assets
Assets
Gross Recognized Derivatives	32,129	28,562
Amounts Offset	(18,885)	(15,127)
Net Recognized Derivatives	13,244	13,435
Other	410	350
Amount Reported on Balance Sheet	13,654	13,785
Investments and Other Assets
Assets
Gross Recognized Derivatives	18,418	24,810
Amounts Offset	(4,340)	(7,190)
Net Recognized Derivatives	14,078	17,620
Other	1,230	0
Amount Reported on Balance Sheet	15,308	17,620
Current Liabilities
Liabilities
Gross Recognized Derivatives	(99,301)	(86,062)
Amounts Offset	32,785	33,829
Net Recognized Derivatives	(66,516)	(52,233)
Other	(8,121)	(7,443)
Amount Reported on Balance Sheet	(74,637)	(59,676)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(100,700)	(82,990)
Amounts Offset	4,340	32,388
Net Recognized Derivatives	(96,360)	(50,602)
Other	0	0
Amount Reported on Balance Sheet	$ (96,360)	$ (50,602)