Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Assets
Gross Recognized Derivatives	$ 109,614,000	$ 4,749,000
Liabilities
Amount Reported on Balance Sheets	(1,512,000)	(18,619,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	110,982,000	9,020,000
Amounts Offset	(1,368,000)	(4,271,000)
Net Recognized Derivatives	109,614,000	4,749,000
Other	0	0
Amount Reported on Balance Sheets	109,614,000	4,749,000
Liabilities
Gross Recognized Derivatives	(1,595,000)	(21,605,000)
Amounts Offset	1,368,000	4,271,000
Net Recognized Derivatives	(227,000)	(17,334,000)
Other	(1,285,000)	(1,285,000)
Amount Reported on Balance Sheets	(1,512,000)	(18,619,000)
Assets and Liabilities
Gross Recognized Derivatives	109,387,000	(12,585,000)
Amounts Offset	0	0
Net Recognized Derivatives	109,387,000	(12,585,000)
Other	(1,285,000)	(1,285,000)
Amount Reported on Balance Sheets	108,102,000	(13,870,000)
Cash collateral received from counterparties	1,285,000	1,285,000
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	83,677,000	5,870,000
Amounts Offset	(1,368,000)	(2,939,000)
Net Recognized Derivatives	82,309,000	2,931,000
Other	0	0
Amount Reported on Balance Sheets	82,309,000	2,931,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	27,305,000	3,150,000
Amounts Offset	0	(1,332,000)
Net Recognized Derivatives	27,305,000	1,818,000
Other	0	0
Amount Reported on Balance Sheets	27,305,000	1,818,000
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(1,595,000)	(9,211,000)
Amounts Offset	1,368,000	2,939,000
Net Recognized Derivatives	(227,000)	(6,272,000)
Other	(1,285,000)	(1,285,000)
Amount Reported on Balance Sheets	(1,512,000)	(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	0	(12,394,000)
Amounts Offset	0	1,332,000
Net Recognized Derivatives	0	(11,062,000)
Other	0	0
Amount Reported on Balance Sheets	0	(11,062,000)
Assets and Liabilities
Cash collateral received from counterparties	$ 0	$ 0