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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rates	$ 3,459	$ 2,002	$ 20,297	$ 13,963
Tax effect of regulatory treatment of utility plant differences	(3,204)	(2,415)	(6,444)	(4,817)
State income tax expense	280	(325)	832	902
Non-plant excess deferred turnaround	0	(8,390)	0	(8,476)
Settlement of equity awards	(21)	0	909	165
Other	1,884	1,209	(1,032)	(1,124)
Total income tax expense	$ 2,398	$ (7,919)	$ 14,562	$ 613
Federal income taxes at statutory rates	21.00%	21.00%	21.00%	21.00%
Tax effect of regulatory treatment of utility plant differences	(19.50%)	(25.30%)	(6.70%)	(7.20%)
State income tax expense	1.70%	(3.40%)	0.90%	1.40%
Non-plant excess deferred turnaround	0.00%	(88.00%)	0.00%	(12.70%)
Settlement of equity awards	(0.10%)	0.00%	1.00%	0.10%
Other	11.50%	12.70%	(1.10%)	(1.70%)
Total income tax expense	14.60%	(83.00%)	15.10%	0.90%