Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Assets
Gross Recognized Derivatives	$ 17,304,000	$ 515,000
Liabilities
Amount Reported on Balance Sheets	(13,358,000)	(72,132,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	20,513,000
Amounts Offset	(3,209,000)
Net Recognized Derivatives	17,304,000
Other	0	405,000
Amount Reported on Balance Sheets	17,304,000
Liabilities
Gross Recognized Derivatives	(15,282,000)	(71,421,000)
Amounts Offset	3,209,000	474,000
Net Recognized Derivatives	(12,073,000)	(70,947,000)
Other	(1,285,000)	(1,185,000)
Amount Reported on Balance Sheets	(13,358,000)	(72,132,000)
Assets and Liabilities
Gross Recognized Derivatives	5,231,000	(70,837,000)
Amounts Offset	0	0
Net Recognized Derivatives	5,231,000	(70,837,000)
Other	(1,285,000)	(780,000)
Amount Reported on Balance Sheets	3,946,000	(71,617,000)
Cash collateral received from counterparties	1,285,000	1,185,000
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	15,505,000	584,000
Amounts Offset	(1,630,000)	(474,000)
Net Recognized Derivatives	13,875,000	110,000
Other	0	405,000
Amount Reported on Balance Sheets	13,875,000	515,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	5,008,000
Amounts Offset	(1,579,000)
Net Recognized Derivatives	3,429,000
Other	0
Amount Reported on Balance Sheets	3,429,000
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(4,611,000)	(38,235,000)
Amounts Offset	1,630,000	474,000
Net Recognized Derivatives	(2,981,000)	(37,761,000)
Other	(1,285,000)	(1,185,000)
Amount Reported on Balance Sheets	(4,266,000)	(38,946,000)
Assets and Liabilities
Cash collateral received from counterparties	1,285,000	1,185,000
Commodity Contracts | Deferred credits and other
Liabilities
Gross Recognized Derivatives	(10,671,000)	(33,186,000)
Amounts Offset	1,579,000	0
Net Recognized Derivatives	(9,092,000)	(33,186,000)
Other	0	0
Amount Reported on Balance Sheets	(9,092,000)	(33,186,000)
Assets and Liabilities
Cash collateral received from counterparties	$ 0	$ 0