Income Taxes - Summary of Significant Factors Impact on Difference Between Effective Tax Rate and Federal Statutory Rate (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rates, amount	$ 1,204	$ 1,041	$ 15,167	$ 36,554
Tax effect of regulatory treatment of utility plant differences, amount	(2,427)	(2,139)	(7,244)	(6,358)
State income tax expense, amount	(363)	(800)	539	851
Settlement of prior year tax returns, amount	524	(604)	1,565	1,995
Acquisition costs, amount	0	0	0	(1,712)
Non-plant excess deferred turnaround, amount	0	(20)	(8,476)	(5,621)
Tax loss on sale of METALfx, amount	0	(13)	0	(1,272)
Valuation allowance, amount	0	0	0	1,245
Settlement of equity awards, amount	0	0	165	612
Other, amount	1,921	2,404	(244)	1,851
Total income tax expense, amount	$ 859	$ (131)	$ 1,472	$ 28,145
Federal income taxes at statutory rates, percent	21.00%	21.00%	21.00%	21.00%
Tax effect of regulatory treatment of utility plant differences, percent	(42.30%)	(43.10%)	(10.00%)	(3.70%)
State income tax expense, percent	(6.30%)	(16.10%)	0.70%	0.50%
Settlement of prior year tax returns, percent	9.10%	(12.20%)	2.20%	1.10%
Acquisition costs, percent	0.00%	0.00%	0.00%	(1.00%)
Non-plant excess deferred turnaround, percent	0.00%	(0.40%)	(11.80%)	(3.20%)
Tax loss on sale of METALfx, percent	0.00%	(0.20%)	0.00%	(0.70%)
Valuation allowance, percent	0.00%	0.00%	0.00%	0.70%
Settlement of equity awards, percent	0.00%	0.00%	0.20%	0.40%
Other, percent	33.50%	48.40%	(0.30%)	1.10%
Total income tax expense, percent	15.00%	(2.60%)	2.00%	16.20%