Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Assets
Gross Recognized Derivatives	$ 4,749,000	$ 515,000
Liabilities
Amount Reported on Balance Sheet	(18,619,000)	(72,132,000)
Commodity Contracts
Assets
Gross Recognized Derivatives	9,020,000
Amounts Offset	(4,271,000)
Net Recognized Derivatives	4,749,000
Other	0	405,000
Amount Reported on Balance Sheet	4,749,000
Liabilities
Gross Recognized Derivatives	(21,605,000)	(71,421,000)
Amounts Offset	4,271,000	474,000
Net Recognized Derivatives	(17,334,000)	(70,947,000)
Other	(1,285,000)	(1,185,000)
Amount Reported on Balance Sheet	(18,619,000)	(72,132,000)
Assets and Liabilities
Gross Recognized Derivatives	(12,585,000)	(70,837,000)
Amounts Offset	0	0
Net Recognized Derivatives	(12,585,000)	(70,837,000)
Other	(1,285,000)	(780,000)
Amount Reported on Balance Sheet	(13,870,000)	(71,617,000)
Commodity Contracts | Current Assets
Assets
Gross Recognized Derivatives	5,870,000	584,000
Amounts Offset	(2,939,000)	(474,000)
Net Recognized Derivatives	2,931,000	110,000
Other	0	405,000
Amount Reported on Balance Sheet	2,931,000	515,000
Commodity Contracts | Investments and Other Assets
Assets
Gross Recognized Derivatives	3,150,000
Amounts Offset	(1,332,000)
Net Recognized Derivatives	1,818,000
Other	0
Amount Reported on Balance Sheet	1,818,000
Commodity Contracts | Current Liabilities
Liabilities
Gross Recognized Derivatives	(9,211,000)	(38,235,000)
Amounts Offset	2,939,000	474,000
Net Recognized Derivatives	(6,272,000)	(37,761,000)
Other	(1,285,000)	(1,185,000)
Amount Reported on Balance Sheet	(7,557,000)	(38,946,000)
Commodity Contracts | Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(12,394,000)	(33,186,000)
Amounts Offset	1,332,000	0
Net Recognized Derivatives	(11,062,000)	(33,186,000)
Other	0	0
Amount Reported on Balance Sheet	$ (11,062,000)	$ (33,186,000)