Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Current Income Tax Expense
Federal		$ 2,775	$ (2,777)	$ (2,252)
State		(96)	2,162	(491)
Other		(47)	(47)	(47)
Total current income tax expense (benefit)		2,632	(662)	(2,790)
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	26,599	24,200	23,904
Income Tax Expense from Continuing Operations		29,231	23,538	21,114
Discontinued Operation, Tax Effect of Gain (Loss) from Disposal of Discontinued Operation		0	153	1,416
Income Tax Expense (Benefit), Continuing Operations, Discontinued Operations		29,231	23,691	22,530
Deferred State and Local Income Tax Expense (Benefit)		8,200	4,900	4,700
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		24,074	23,224	25,907
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		1,857	(714)	79
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		(655)	(75)	(454)
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		(351)	156	(278)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		97	5,107	(3,772)
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ 1,577	$ (3,498)	$ 2,422

[1] (1) Includes $8.2 million, $4.9 million, and $4.7 million of deferred state income taxes for the years 2021, 2020 and 2019, respectively.