Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Assets
Gross Recognized Derivatives	$ 535,000	$ 1,113,000
Liabilities
Amount Reported on Balance Sheets	(80,426,000)	(60,037,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	3,634,000	3,142,000
Amounts Offset	(2,939,000)	(2,185,000)
Net Recognized Derivatives	695,000	957,000
Other	160,000	(156,000)
Amount Reported on Balance Sheets	535,000	1,113,000
Liabilities
Gross Recognized Derivatives	(82,180,000)	(60,912,000)
Amounts Offset	2,939,000	2,185,000
Net Recognized Derivatives	(79,241,000)	(58,727,000)
Other	(1,185,000)	(1,310,000)
Amount Reported on Balance Sheets	(80,426,000)	(60,037,000)
Assets and Liabilities
Gross Recognized Derivatives	(78,546,000)	(57,770,000)
Amounts Offset	0	0
Net Recognized Derivatives	(78,546,000)	(57,770,000)
Other	(1,345,000)	(1,154,000)
Amount Reported on Balance Sheets	(79,891,000)	(58,924,000)
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	3,558,000	3,106,000
Amounts Offset	(2,939,000)	(2,149,000)
Net Recognized Derivatives	619,000	957,000
Other	160,000	(156,000)
Amount Reported on Balance Sheets	459,000	1,113,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	76,000	36,000
Amounts Offset	0	(36,000)
Net Recognized Derivatives	76,000	0
Other	0	0
Amount Reported on Balance Sheets	76,000	0
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(60,588,000)	(36,345,000)
Amounts Offset	2,939,000	2,149,000
Net Recognized Derivatives	(57,649,000)	(34,196,000)
Other	(1,185,000)	(1,310,000)
Amount Reported on Balance Sheets	(58,834,000)	(35,506,000)
Commodity Contracts | Deferred credits and other
Liabilities
Gross Recognized Derivatives	(21,592,000)	(24,567,000)
Amounts Offset	0	36,000
Net Recognized Derivatives	(21,592,000)	(24,531,000)
Other	0	0
Amount Reported on Balance Sheets	$ (21,592,000)	$ (24,531,000)