Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Commodity Contracts
Assets
Gross Recognized Derivatives	$ 53,356	$ 53,367
Amounts Offset	(26,017)	(22,317)
Net Recognized Derivatives	27,339	31,050
Other	672	355
Amount Reported on Balance Sheet	28,011	31,405
Liabilities
Gross Recognized Derivatives	(207,387)	(169,045)
Amounts Offset	44,077	66,217
Net Recognized Derivatives	(163,310)	(102,828)
Other	(4,379)	(7,443)
Amount Reported on Balance Sheet	(167,689)	(110,271)
Assets and Liabilities
Gross Recognized Derivatives	(154,031)	(115,678)
Amounts Offset	18,060	43,900
Net Recognized Derivatives	(135,971)	(71,778)
Other	(3,707)	(7,088)
Amount Reported on Balance Sheet	(139,678)	(78,866)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	37,396	28,557
Amounts Offset	(22,163)	(15,127)
Net Recognized Derivatives	15,233	13,430
Other	672	355
Amount Reported on Balance Sheet	15,905	13,785
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	15,960	24,810
Amounts Offset	(3,854)	(7,190)
Net Recognized Derivatives	12,106	17,620
Other	0	0
Amount Reported on Balance Sheet	12,106	17,620
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(113,560)	(86,055)
Amounts Offset	40,223	33,829
Net Recognized Derivatives	(73,337)	(52,226)
Other	(4,379)	(7,443)
Amount Reported on Balance Sheet	(77,716)	(59,669)
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(93,827)	(82,990)
Amounts Offset	3,854	32,388
Net Recognized Derivatives	(89,973)	(50,602)
Other	0	0
Amount Reported on Balance Sheet	(89,973)	(50,602)
Designated as Hedging Instruments
Liabilities
Amount Reported on Balance Sheet	$ (3,000)	$ (4,000)