Accounting For Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rates	$ 28,673	$ 47,909	$ 34,158
Tax effect of regulatory treatment of utility plant differences	(12,893)	(9,967)	(8,153)
State income tax expense	814	1,465	1,191
Acquisition costs		(1,712)	329
Non-plant excess deferred turnaround	(8,476)	(5,690)
Tax loss on sale of METALfx		(1,272)
Customer refunds related to prior years at 35 percent	(1,189)
Other	122	641	(1,465)
Total income tax expense	$ 7,051	$ 31,374	$ 26,060
Federal statutory tax rate	21.00%	21.00%	21.00%	35.00%
Tax effect of regulatory treatment of utility plant differences	(9.40%)	(4.30%)	(5.00%)
State income tax expense	0.60%	0.60%	0.70%
Acquisition costs		(0.70%)	0.20%
Non-plant excess deferred turnaround	(6.20%)	(2.50%)
Tax loss on sale of METALfx		(0.60%)
Customer refunds related to prior years at 35 percent	(0.90%)
Other	0.10%	0.30%	(0.90%)
Effective Income Tax Rate Reconciliation, Percent	5.20%	13.80%	16.00%