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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rates	$ 1,876	$ 1,204	$ 22,173	$ 15,167
Tax effect of regulatory treatment of utility plant differences	(3,277)	(2,427)	(9,721)	(7,244)
State income tax expense	143	(363)	975	539
Non-plant excess deferred turnaround	0	0	0	(8,476)
Settlement of prior year tax returns	(400)	524	(400)	1,565
Flow through related to deduction of meters and mixed service costs	(5,277)	0	(5,277)	0
Settlement of equity awards	0	0	909	165
Other	1,503	1,921	471	(244)
Total income tax expense (benefit)	$ (5,432)	$ 859	$ 9,130	$ 1,472
Federal income taxes at statutory rates	21.00%	21.00%	21.00%	21.00%
Tax effect of regulatory treatment of utility plant differences	(36.70%)	(42.30%)	(9.20%)	(10.00%)
State income tax expense	1.60%	(6.30%)	0.90%	0.70%
Non-plant excess deferred turnaround	0.00%	0.00%	0.00%	(11.80%)
Settlemet of prior year tax returns percent	(4.50%)	9.10%	(0.40%)	2.20%
Flow through related to deduction of meters and mixed service costs percent	(59.00%)	0.00%	(5.00%)	0.00%
Settlement of equity awards	0.00%	0.00%	0.90%	0.20%
Other	16.80%	33.50%	0.40%	(0.30%)
Total income tax expense (benefit)	(60.80%)	15.00%	8.60%	2.00%