Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Assets
Gross Recognized Derivatives	$ 17,244,000	$ 54,798,000
Amounts Offset	(16,952,000)	(35,103,000)
Net Recognized Derivatives	292,000	19,695,000
Other	70,000	0
Derivative assets	362,000	19,695,000
Liabilities
Gross Recognized Derivatives	(112,797,000)	(104,123,000)
Amounts Offset	20,152,000	35,103,000
Net Recognized Derivatives	(92,645,000)	(69,020,000)
Other	(2,554,000)	(4,054,000)
Amount Reported on Balance Sheet	(95,199,000)	(73,074,000)
Assets and Liabilities
Gross Recognized Derivatives	(95,553,000)	(49,325,000)
Amounts Offset	3,200,000	0
Net Recognized Derivatives	(92,353,000)	(49,325,000)
Other	(2,484,000)	(4,054,000)
Amount Reported on Balance Sheet	(94,837,000)	(53,379,000)
Current Assets
Assets
Gross Recognized Derivatives	15,624,000	48,094,000
Amounts Offset	(15,387,000)	(28,400,000)
Net Recognized Derivatives	237,000	19,694,000
Other	70,000	0
Derivative assets	307,000	19,694,000
Investments and Other Assets
Assets
Gross Recognized Derivatives	1,620,000	6,704,000
Amounts Offset	(1,565,000)	(6,703,000)
Net Recognized Derivatives	55,000	1,000
Other	0	0
Derivative assets	55,000	1,000
Current Liabilities
Liabilities
Gross Recognized Derivatives	(64,646,000)	(50,182,000)
Amounts Offset	18,587,000	28,400,000
Net Recognized Derivatives	(46,059,000)	(21,782,000)
Other	(2,554,000)	(4,054,000)
Amount Reported on Balance Sheet	(48,613,000)	(25,836,000)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(48,151,000)	(53,941,000)
Amounts Offset	1,565,000	6,703,000
Net Recognized Derivatives	(46,586,000)	(47,238,000)
Other	0	0
Amount Reported on Balance Sheet	$ (46,586,000)	$ (47,238,000)