Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Commodity Contracts
Assets
Gross Recognized Derivatives	$ 54,798,000	$ 53,356,000
Amounts Offset	(35,103,000)	(26,017,000)
Net Recognized Derivatives	19,695,000	27,339,000
Other	0	672,000
Amount Reported on Balance Sheet	19,695,000	28,011,000
Liabilities
Gross Recognized Derivatives	(104,123,000)	(207,387,000)
Amounts Offset	35,103,000	44,077,000
Net Recognized Derivatives	(69,020,000)	(163,310,000)
Other	(4,054,000)	(4,379,000)
Amount Reported on Balance Sheet	(73,074,000)	(167,689,000)
Assets and Liabilities
Gross Recognized Derivatives	(49,325,000)	(154,031,000)
Amounts Offset	0	18,060,000
Net Recognized Derivatives	(49,325,000)	(135,971,000)
Other	(4,054,000)	(3,707,000)
Amount Reported on Balance Sheet	(53,379,000)	(139,678,000)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	48,094,000	37,396,000
Amounts Offset	(28,400,000)	(22,163,000)
Net Recognized Derivatives	19,694,000	15,233,000
Other	0	672,000
Amount Reported on Balance Sheet	19,694,000	15,905,000
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	6,704,000	15,960,000
Amounts Offset	(6,703,000)	(3,854,000)
Net Recognized Derivatives	1,000	12,106,000
Other	0	0
Amount Reported on Balance Sheet	1,000	12,106,000
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(50,182,000)	(113,560,000)
Amounts Offset	28,400,000	40,223,000
Net Recognized Derivatives	(21,782,000)	(73,337,000)
Other	(4,054,000)	(4,379,000)
Amount Reported on Balance Sheet	(25,836,000)	(77,716,000)
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(53,941,000)	(93,827,000)
Amounts Offset	6,703,000	3,854,000
Net Recognized Derivatives	(47,238,000)	(89,973,000)
Other	0	0
Amount Reported on Balance Sheet	(47,238,000)	(89,973,000)
Designated as Hedging Instruments
Liabilities
Amount Reported on Balance Sheet	$ (2,000,000)	$ (3,000,000)