Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commodity Contracts
Assets
Gross Recognized Derivatives	$ 53,372	$ 49,951
Amounts Offset	(22,317)	(8,974)
Net Recognized Derivatives	31,055	40,977
Other	350	7
Amount Reported on Balance Sheet	31,405	40,984
Liabilities
Gross Recognized Derivatives	(169,052)	(122,663)
Amounts Offset	66,217	27,974
Net Recognized Derivatives	(102,835)	(94,689)
Other	(7,443)	(7,518)
Amount Reported on Balance Sheet	(110,278)	(102,207)
Assets and Liabilities
Gross Recognized Derivatives	(115,680)	(72,712)
Amounts Offset	43,900	19,000
Net Recognized Derivatives	(71,780)	(53,712)
Other	(7,093)	(7,511)
Amount Reported on Balance Sheet	(78,873)	(61,223)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	28,562	24,587
Amounts Offset	(15,127)	(7,425)
Net Recognized Derivatives	13,435	17,162
Other	350	7
Amount Reported on Balance Sheet	13,785	17,169
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	24,810	25,364
Amounts Offset	(7,190)	(1,549)
Net Recognized Derivatives	17,620	23,815
Other	0	0
Amount Reported on Balance Sheet	17,620	23,815
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(86,062)	(50,540)
Amounts Offset	33,829	26,166
Net Recognized Derivatives	(52,233)	(24,374)
Other	(7,443)	(7,518)
Amount Reported on Balance Sheet	(59,676)	(31,892)
Assets and Liabilities
Amounts Offset	43,900	19,000
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(82,990)	(72,123)
Amounts Offset	32,388	1,808
Net Recognized Derivatives	(50,602)	(70,315)
Other	0	0
Amount Reported on Balance Sheet	(50,602)	(70,315)
Designated as Hedging Instruments
Liabilities
Amount Reported on Balance Sheet	$ (4,000)	$ (5,000)