Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Assets
Gross Recognized Derivatives	$ 195,277,000	$ 4,749,000
Liabilities
Amount Reported on Balance Sheets	(1,652,000)	(18,619,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	197,884,000	9,020,000
Amounts Offset	(2,683,000)	(4,271,000)
Net Recognized Derivatives	195,201,000	4,749,000
Other	76,000	0
Amount Reported on Balance Sheets	195,277,000	4,749,000
Liabilities
Gross Recognized Derivatives	(2,700,000)	(21,605,000)
Amounts Offset	2,683,000	4,271,000
Net Recognized Derivatives	(17,000)	(17,334,000)
Other	(1,635,000)	(1,285,000)
Amount Reported on Balance Sheets	(1,652,000)	(18,619,000)
Assets and Liabilities
Gross Recognized Derivatives	195,184,000	(12,585,000)
Amounts Offset	0	0
Net Recognized Derivatives	195,184,000	(12,585,000)
Other	(1,559,000)	(1,285,000)
Amount Reported on Balance Sheets	193,625,000	(13,870,000)
Cash collateral received from counterparties	1,635,000	1,285,000
Commodity Contracts | Current assets
Assets
Gross Recognized Derivatives	129,202,000	5,870,000
Amounts Offset	(2,683,000)	(2,939,000)
Net Recognized Derivatives	126,519,000	2,931,000
Other	76,000	0
Amount Reported on Balance Sheets	126,595,000	2,931,000
Commodity Contracts | Investments and other assets
Assets
Gross Recognized Derivatives	68,682,000	3,150,000
Amounts Offset	0	(1,332,000)
Net Recognized Derivatives	68,682,000	1,818,000
Other	0	0
Amount Reported on Balance Sheets	68,682,000	1,818,000
Commodity Contracts | Current liabilities
Liabilities
Gross Recognized Derivatives	(2,700,000)	(9,211,000)
Amounts Offset	2,683,000	2,939,000
Net Recognized Derivatives	(17,000)	(6,272,000)
Other	(1,635,000)	(1,285,000)
Amount Reported on Balance Sheets	(1,652,000)	(7,557,000)
Assets and Liabilities
Cash collateral received from counterparties	1,635,000	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