Income Taxes - Summary of Significant Factors Impact on Difference Between Effective Tax Rate and Federal Statutory Rate (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rates, amount	$ 2,002	$ 4,874	$ 13,963	$ 35,513
Tax effect of regulatory treatment of utility plant differences, amount	(2,415)	(2,139)	(4,817)	(4,219)
State income tax expense, amount	(325)	(8)	902	1,651
Acquisition costs, amount	0	112	0	(1,712)
Non-plant excess deferred turnaround, amount	(8,390)	(5,091)	(8,476)	(5,601)
Tax loss on sale of METALfx, amount	0	(1,259)	0	(1,259)
Valuation allowance, amount	0	1,245	0	1,245
Settlement of equity awards, amount	0	0	165	612
Other, amount	1,209	525	(1,124)	2,046
Total income tax expense, amount	$ (7,919)	$ (1,741)	$ 613	$ 28,276
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	(25.30%)	(9.20%)	(7.20%)	(2.50%)
State income tax expense, percent	(3.40%)	0.00%	1.40%	1.00%
Acquisition costs, percent	0.00%	0.50%	0.00%	(1.00%)
Non-plant excess deferred turnaround, percent	(88.00%)	(21.90%)	(12.70%)	(3.30%)
Tax loss on sale of METALfx, percent	0.00%	(5.40%)	0.00%	(0.80%)
Valuation allowance, percent	0.00%	5.30%	0.00%	0.70%
Settlement of equity awards, percent	0.00%	0.00%	0.10%	0.40%
Other, percent	12.70%	2.20%	(1.70%)	1.20%
Total income tax expense, percent	(83.00%)	(7.50%)	0.90%	16.70%