Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Assets
Gross Recognized Derivatives	$ 817,000	$ 1,113,000
Liabilities
Amount Reported on Balance Sheets	(71,654,000)	(60,037,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	1,776,000	3,142,000
Amounts Offset	(1,266,000)	(2,185,000)
Net Recognized Derivatives	510,000	957,000
Other	307,000	156,000
Amount Reported on Balance Sheets	817,000	1,113,000
Liabilities
Gross Recognized Derivatives	(71,734,000)	(60,912,000)
Amounts Offset	1,266,000	2,185,000
Net Recognized Derivatives	(70,468,000)	(58,727,000)
Other	(1,186,000)	(1,310,000)
Amount Reported on Balance Sheets	(71,654,000)	(60,037,000)
Assets and Liabilities
Gross Recognized Derivatives	(69,958,000)	(57,770,000)
Amounts Offset	0	0
Net Recognized Derivatives	(69,958,000)	(57,770,000)
Other	(879,000)	(1,154,000)
Amount Reported on Balance Sheets	(70,837,000)	(58,924,000)
Cash collateral received from counterparties	1,186,000	1,310,000
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	1,776,000	3,106,000
Amounts Offset	(1,266,000)	(2,149,000)
Net Recognized Derivatives	510,000	957,000
Other	307,000	156,000
Amount Reported on Balance Sheets	817,000	1,113,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives		36,000
Amounts Offset		(36,000)
Net Recognized Derivatives		0
Other		0
Amount Reported on Balance Sheets		0
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(44,429,000)	(36,345,000)
Amounts Offset	1,266,000	2,149,000
Net Recognized Derivatives	(43,163,000)	(34,196,000)
Other	(1,186,000)	(1,310,000)
Amount Reported on Balance Sheets	(44,349,000)	(35,506,000)
Assets and Liabilities
Cash collateral received from counterparties	1,186,000	1,310,000
Commodity Contracts | Deferred credits and other
Liabilities
Gross Recognized Derivatives	(27,305,000)	(24,567,000)
Amounts Offset	0	36,000
Net Recognized Derivatives	(27,305,000)	(24,531,000)
Other	0	0
Amount Reported on Balance Sheets	(27,305,000)	(24,531,000)
Assets and Liabilities
Cash collateral received from counterparties	$ 0	$ 0