Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Assets
Gross Recognized Derivatives	$ 25,811,000	$ 4,749,000
Liabilities
Amount Reported on Balance Sheet	(9,995,000)	(18,619,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	29,693,000	9,020,000
Amounts Offset	(3,882,000)	(4,271,000)
Net Recognized Derivatives	25,811,000	4,749,000
Other	0	0
Amount Reported on Balance Sheet	25,811,000	4,749,000
Liabilities
Gross Recognized Derivatives	(12,592,000)	(21,605,000)
Amounts Offset	3,882,000	4,271,000
Net Recognized Derivatives	(8,710,000)	(17,334,000)
Other	(1,285,000)	(1,285,000)
Amount Reported on Balance Sheet	(9,995,000)	(18,619,000)
Assets and Liabilities
Gross Recognized Derivatives	17,101,000	(12,585,000)
Amounts Offset	0	0
Net Recognized Derivatives	17,101,000	(12,585,000)
Other	(1,285,000)	(1,285,000)
Amount Reported on Balance Sheet	15,816,000	(13,870,000)
Cash collateral received from counterparties	1,285,000	1,285,000
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	25,703,000	5,870,000
Amounts Offset	(3,092,000)	(2,939,000)
Net Recognized Derivatives	22,611,000	2,931,000
Other	0	0
Amount Reported on Balance Sheet	22,611,000	2,931,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	3,990,000	3,150,000
Amounts Offset	(790,000)	(1,332,000)
Net Recognized Derivatives	3,200,000	1,818,000
Other	0	0
Amount Reported on Balance Sheet	3,200,000	1,818,000
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(4,874,000)	(9,211,000)
Amounts Offset	3,092,000	2,939,000
Net Recognized Derivatives	(1,782,000)	(6,272,000)
Other	(1,285,000)	(1,285,000)
Amount Reported on Balance Sheet	(3,067,000)	(7,557,000)
Assets and Liabilities
Cash collateral received from counterparties	1,285,000	1,285,000
Commodity Contracts | Deferred credits and other
Liabilities
Gross Recognized Derivatives	(7,718,000)	(12,394,000)
Amounts Offset	790,000	1,332,000
Net Recognized Derivatives	(6,928,000)	(11,062,000)
Other	0	0
Amount Reported on Balance Sheet	(6,928,000)	(11,062,000)
Assets and Liabilities
Cash collateral received from counterparties	$ 0	$ 0