Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Commodity Contracts
Assets
Gross Recognized Derivatives	$ 6,719,000	$ 54,798,000
Amounts Offset	(5,037,000)	(35,103,000)
Net Recognized Derivatives	1,682,000	19,695,000
Other	300,000	0
Amount Reported on Balance Sheet	1,982,000	19,695,000
Liabilities
Gross Recognized Derivatives	(97,938,000)	(104,123,000)
Amounts Offset	5,037,000	35,103,000
Net Recognized Derivatives	(92,901,000)	(69,020,000)
Other	(3,521,000)	(4,054,000)
Amount Reported on Balance Sheet	(96,422,000)	(73,074,000)
Assets and Liabilities
Gross Recognized Derivatives	(91,219,000)	(49,325,000)
Amounts Offset	0	0
Net Recognized Derivatives	(91,219,000)	(49,325,000)
Other	(3,221,000)	(4,054,000)
Amount Reported on Balance Sheet	(94,440,000)	(53,379,000)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	5,427,000	48,094,000
Amounts Offset	(3,796,000)	(28,400,000)
Net Recognized Derivatives	1,631,000	19,694,000
Other	300,000	0
Amount Reported on Balance Sheet	1,931,000	19,694,000
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	1,292,000	6,704,000
Amounts Offset	(1,241,000)	(6,703,000)
Net Recognized Derivatives	51,000	1,000
Other	0	0
Amount Reported on Balance Sheet	51,000	1,000
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(59,527,000)	(50,182,000)
Amounts Offset	3,796,000	28,400,000
Net Recognized Derivatives	(55,731,000)	(21,782,000)
Other	(3,521,000)	(4,054,000)
Amount Reported on Balance Sheet	(59,252,000)	(25,836,000)
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(38,411,000)	(53,941,000)
Amounts Offset	1,241,000	6,703,000
Net Recognized Derivatives	(37,170,000)	(47,238,000)
Other	0	0
Amount Reported on Balance Sheet	(37,170,000)	(47,238,000)
Designated as Hedging Instruments
Liabilities
Amount Reported on Balance Sheet	$ 0	$ (2,000,000)