Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Gross Recognized Derivatives	$ 36,613	$ 53,356
Amounts Offset	(11,904)	(26,017)
Net Recognized Derivatives	24,709	27,339
Other	(8)	672
Amount Reported on Balance Sheet	24,701	28,011
Liabilities
Gross Recognized Derivatives	(204,061)	(207,387)
Amounts Offset	29,504	44,077
Net Recognized Derivatives	(174,557)	(163,310)
Other	(4,329)	(4,379)
Amount Reported on Balance Sheet	(178,886)	(167,689)
Assets and Liabilities
Gross Recognized Derivatives	(167,448)	(154,031)
Amounts Offset	17,600	18,060
Net Recognized Derivatives	(149,848)	(135,971)
Other	(4,337)	(3,707)
Amount Reported on Balance Sheet	(154,185)	(139,678)
Current Assets
Assets
Gross Recognized Derivatives	25,365	37,396
Amounts Offset	(9,268)	(22,163)
Net Recognized Derivatives	16,097	15,233
Other	(8)	672
Amount Reported on Balance Sheet	16,089	15,905
Investments and Other Assets
Assets
Gross Recognized Derivatives	11,248	15,960
Amounts Offset	(2,636)	(3,854)
Net Recognized Derivatives	8,612	12,106
Other	0	0
Amount Reported on Balance Sheet	8,612	12,106
Current Liabilities
Liabilities
Gross Recognized Derivatives	(115,822)	(113,560)
Amounts Offset	26,868	40,223
Net Recognized Derivatives	(88,954)	(73,337)
Other	(4,329)	(4,379)
Amount Reported on Balance Sheet	(93,283)	(77,716)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(88,239)	(93,827)
Amounts Offset	2,636	3,854
Net Recognized Derivatives	(85,603)	(89,973)
Other	0	0
Amount Reported on Balance Sheet	$ (85,603)	$ (89,973)