Derivative Accounting (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Commodity Contracts
Assets
Amount Reported on Balance Sheet	$ 49,000
Assets and Liabilities
Gross Recognized Derivatives	(81,325)	(122,252)
Amounts Offset	28,300	49,299
Net Recognized Derivatives	(53,025)	(72,953)
Other	(19,318)	(24,462)
Amount Reported on Balance Sheet	(72,343)	(97,415)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	32,201	42,495
Amounts Offset	(9,605)	(17,797)
Net Recognized Derivatives	22,596	24,698
Other	145	1,001
Amount Reported on Balance Sheet	22,741	25,699
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	27,905	41,563
Amounts Offset	(1,859)	(5,672)
Net Recognized Derivatives	26,046	35,891
Amount Reported on Balance Sheet	26,046	35,891
Commodity Contracts | Total Assets
Assets
Gross Recognized Derivatives	60,106	84,058
Amounts Offset	(11,464)	(23,469)
Net Recognized Derivatives	48,642	60,589
Other	145	1,001
Amount Reported on Balance Sheet	48,787	61,590
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(66,680)	(105,324)
Amounts Offset	32,675	57,046
Net Recognized Derivatives	(34,005)	(48,278)
Other	(19,463)	(25,463)
Amount Reported on Balance Sheet	(53,468)	(73,741)
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(74,751)	(100,986)
Amounts Offset	7,089	15,722
Net Recognized Derivatives	(67,662)	(85,264)
Amount Reported on Balance Sheet	(67,662)	(85,264)
Commodity Contracts | Total Liabilities
Liabilities
Gross Recognized Derivatives	(141,431)	(206,310)
Amounts Offset	39,764	72,768
Net Recognized Derivatives	(101,667)	(133,542)
Other	(19,463)	(25,463)
Amount Reported on Balance Sheet	(121,130)	(159,005)
Designated as Hedging Instruments
Liabilities
Amount Reported on Balance Sheet	$ 5,000	$ 5,000