Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Assets
Gross Recognized Derivatives	$ 1,224,000	$ 1,982,000
Liabilities
Amount Reported on Balance Sheets	(79,730,000)	(96,422,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	2,923,000	6,719,000
Amounts Offset	(2,587,000)	(5,037,000)
Net Recognized Derivatives	336,000	1,682,000
Other	888,000	300,000
Amount Reported on Balance Sheets	1,224,000	1,982,000
Liabilities
Gross Recognized Derivatives	(79,778,000)	(97,938,000)
Amounts Offset	2,587,000	5,037,000
Net Recognized Derivatives	(77,191,000)	(92,901,000)
Other	(2,539,000)	(3,521,000)
Amount Reported on Balance Sheets	(79,730,000)	(96,422,000)
Assets and Liabilities
Gross Recognized Derivatives	(76,855,000)	(91,219,000)
Amounts Offset	0	0
Net Recognized Derivatives	(76,855,000)	(91,219,000)
Other	(1,651,000)	(3,221,000)
Amount Reported on Balance Sheets	(78,506,000)	(94,440,000)
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	2,609,000	5,427,000
Amounts Offset	(2,273,000)	(3,796,000)
Net Recognized Derivatives	336,000	1,631,000
Other	888,000	300,000
Amount Reported on Balance Sheets	1,224,000	1,931,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	314,000	1,292,000
Amounts Offset	(314,000)	(1,241,000)
Net Recognized Derivatives	0	51,000
Other	0	0
Amount Reported on Balance Sheets	0	51,000
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(45,238,000)	(59,527,000)
Amounts Offset	2,273,000	3,796,000
Net Recognized Derivatives	(42,965,000)	(55,731,000)
Other	(2,539,000)	(3,521,000)
Amount Reported on Balance Sheets	(45,504,000)	(59,252,000)
Commodity Contracts | Deferred credits and other
Liabilities
Gross Recognized Derivatives	(34,540,000)	(38,411,000)
Amounts Offset	314,000	1,241,000
Net Recognized Derivatives	(34,226,000)	(37,170,000)
Other	0	0
Amount Reported on Balance Sheets	$ (34,226,000)	$ (37,170,000)