Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Assets
Gross Recognized Derivatives	$ 6,803,000	$ 6,719,000
Amounts Offset	(5,505,000)	(5,037,000)
Net Recognized Derivatives	1,298,000	1,682,000
Other	696,000	300,000
Derivative assets	1,994,000	1,982,000
Liabilities
Gross Recognized Derivatives	(118,444,000)	(97,938,000)
Amounts Offset	5,505,000	5,037,000
Net Recognized Derivatives	(112,939,000)	(92,901,000)
Other	(2,430,000)	(3,521,000)
Amount Reported on Balance Sheet	(115,369,000)	(96,422,000)
Assets and Liabilities
Gross Recognized Derivatives	(111,641,000)	(91,219,000)
Amounts Offset	0	0
Net Recognized Derivatives	(111,641,000)	(91,219,000)
Other	(1,734,000)	(3,221,000)
Amount Reported on Balance Sheet	(113,375,000)	(94,440,000)
Current Assets
Assets
Gross Recognized Derivatives	5,984,000	5,427,000
Amounts Offset	(4,686,000)	(3,796,000)
Net Recognized Derivatives	1,298,000	1,631,000
Other	696,000	300,000
Derivative assets	1,994,000	1,931,000
Investments and Other Assets
Assets
Gross Recognized Derivatives	819,000	1,292,000
Amounts Offset	(819,000)	(1,241,000)
Net Recognized Derivatives	0	51,000
Other	0	0
Derivative assets	0	51,000
Current Liabilities
Liabilities
Gross Recognized Derivatives	(69,999,000)	(59,527,000)
Amounts Offset	4,686,000	3,796,000
Net Recognized Derivatives	(65,313,000)	(55,731,000)
Other	(2,430,000)	(3,521,000)
Amount Reported on Balance Sheet	(67,743,000)	(59,252,000)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(48,445,000)	(38,411,000)
Amounts Offset	819,000	1,241,000
Net Recognized Derivatives	(47,626,000)	(37,170,000)
Other	0	0
Amount Reported on Balance Sheet	$ (47,626,000)	$ (37,170,000)