Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Current Income Tax Expense
Federal		$ 14,736	$ 8,284	$ 2,775
State		5,496	1,948	(96)
Other		(47)	(47)	(47)
Total current income tax expense (benefit)		20,185	10,185	2,632
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	7,893	23,647	26,599
Total Income Tax		28,078	33,832	29,231
Deferred State and Local Income Tax Expense (Benefit)		100	7,800	8,200
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		17,797	14,968	24,074
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		(7,739)	8,923	1,857
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		(974)	1,109	(655)
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		(351)	(351)	(351)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		(370)	2	97
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ (470)	$ (1,004)	$ 1,577

[1] (1) Includes less than $0.1 million, $7.8 million, and $8.2 million of deferred state income taxes for the years 2023, 2022 and 2021, respectively.