Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets
Gross Recognized Derivatives	$ 44,653	$ 53,356
Amounts Offset	(23,220)	(26,017)
Net Recognized Derivatives	21,433	27,339
Other	707	672
Amount Reported on Balance Sheet	22,140	28,011
Liabilities
Gross Recognized Derivatives	(126,440)	(207,387)
Amounts Offset	23,220	44,077
Net Recognized Derivatives	(103,220)	(163,310)
Other	(4,330)	(4,379)
Amount Reported on Balance Sheet	(107,550)	(167,689)
Assets and Liabilities
Gross Recognized Derivatives	(81,787)	(154,031)
Amounts Offset	0	18,060
Net Recognized Derivatives	(81,787)	(135,971)
Other	(3,623)	(3,707)
Amount Reported on Balance Sheet	(85,410)	(139,678)
Current Assets
Assets
Gross Recognized Derivatives	30,393	37,396
Amounts Offset	(14,424)	(22,163)
Net Recognized Derivatives	15,969	15,233
Other	707	672
Amount Reported on Balance Sheet	16,676	15,905
Investments and Other Assets
Assets
Gross Recognized Derivatives	14,260	15,960
Amounts Offset	(8,796)	(3,854)
Net Recognized Derivatives	5,464	12,106
Other	0	0
Amount Reported on Balance Sheet	5,464	12,106
Current Liabilities
Liabilities
Gross Recognized Derivatives	(65,432)	(113,560)
Amounts Offset	14,424	40,223
Net Recognized Derivatives	(51,008)	(73,337)
Other	(4,330)	(4,379)
Amount Reported on Balance Sheet	(55,338)	(77,716)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(61,008)	(93,827)
Amounts Offset	8,796	3,854
Net Recognized Derivatives	(52,212)	(89,973)
Other	0	0
Amount Reported on Balance Sheet	$ (52,212)	$ (89,973)