Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Assets
Gross Recognized Derivatives	$ 11,901,000	$ 54,798,000
Amounts Offset	(11,676,000)	(35,103,000)
Net Recognized Derivatives	225,000	19,695,000
Other	1,825,000	0
Derivative assets	2,050,000	19,695,000
Liabilities
Gross Recognized Derivatives	(95,491,000)	(104,123,000)
Amounts Offset	11,676,000	35,103,000
Net Recognized Derivatives	(83,815,000)	(69,020,000)
Other	(2,429,000)	(4,054,000)
Amount Reported on Balance Sheet	(86,244,000)	(73,074,000)
Assets and Liabilities
Gross Recognized Derivatives	(83,590,000)	(49,325,000)
Amounts Offset	0	0
Net Recognized Derivatives	(83,590,000)	(49,325,000)
Other	(604,000)	(4,054,000)
Amount Reported on Balance Sheet	(84,194,000)	(53,379,000)
Current Assets
Assets
Gross Recognized Derivatives	9,764,000	48,094,000
Amounts Offset	(9,623,000)	(28,400,000)
Net Recognized Derivatives	141,000	19,694,000
Other	217,000	0
Derivative assets	358,000	19,694,000
Investments and Other Assets
Assets
Gross Recognized Derivatives	2,137,000	6,704,000
Amounts Offset	(2,053,000)	(6,703,000)
Net Recognized Derivatives	84,000	1,000
Other	1,608,000	0
Derivative assets	1,692,000	1,000
Current Liabilities
Liabilities
Gross Recognized Derivatives	(57,663,000)	(50,182,000)
Amounts Offset	9,623,000	28,400,000
Net Recognized Derivatives	(48,040,000)	(21,782,000)
Other	(2,429,000)	(4,054,000)
Amount Reported on Balance Sheet	(50,469,000)	(25,836,000)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(37,828,000)	(53,941,000)
Amounts Offset	2,053,000	6,703,000
Net Recognized Derivatives	(35,775,000)	(47,238,000)
Other	0	0
Amount Reported on Balance Sheet	$ (35,775,000)	$ (47,238,000)