Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Assets
Gross Recognized Derivatives	$ 898,000	$ 1,113,000
Liabilities
Amount Reported on Balance Sheets	(48,133,000)	(60,037,000)
Current assets
Assets
Other	38,000
Commodity Contracts
Assets
Gross Recognized Derivatives	4,674,000	3,142,000
Amounts Offset	(3,738,000)	(2,185,000)
Net Recognized Derivatives	936,000	957,000
Other	(38,000)	156,000
Amount Reported on Balance Sheets	898,000	1,113,000
Liabilities
Gross Recognized Derivatives	(50,212,000)	(60,912,000)
Amounts Offset	3,738,000	2,185,000
Net Recognized Derivatives	(46,474,000)	(58,727,000)
Other	(1,659,000)	(1,310,000)
Amount Reported on Balance Sheets	(48,133,000)	(60,037,000)
Assets and Liabilities
Gross Recognized Derivatives	(45,538,000)	(57,770,000)
Amounts Offset	0	0
Net Recognized Derivatives	(45,538,000)	(57,770,000)
Other	(1,697,000)	(1,154,000)
Amount Reported on Balance Sheets	(47,235,000)	(58,924,000)
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	3,658,000	3,106,000
Amounts Offset	(2,857,000)	(2,149,000)
Net Recognized Derivatives	801,000	957,000
Other	(38,000)	156,000
Amount Reported on Balance Sheets	763,000	1,113,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	1,016,000	36,000
Amounts Offset	(881,000)	(36,000)
Net Recognized Derivatives	135,000	0
Other	0	0
Amount Reported on Balance Sheets	135,000	0
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(34,487,000)	(36,345,000)
Amounts Offset	2,857,000	2,149,000
Net Recognized Derivatives	(31,630,000)	(34,196,000)
Other	(1,659,000)	(1,310,000)
Amount Reported on Balance Sheets	(33,289,000)	(35,506,000)
Commodity Contracts | Deferred credits and other
Liabilities
Gross Recognized Derivatives	(15,725,000)	(24,567,000)
Amounts Offset	881,000	36,000
Net Recognized Derivatives	(14,844,000)	(24,531,000)
Other	0	0
Amount Reported on Balance Sheets	$ (14,844,000)	$ (24,531,000)