Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current Income Tax Expense
Federal		$ (2,777)	$ (2,252)	$ (361)
State		2,162	(491)	617
Other		(47)	(47)	(47)
Total current income tax expense (benefit)		(662)	(2,790)	209
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	24,200	23,904	20,914
Income Tax Expense from Continuing Operations		23,538	21,114	21,123
Discontinued Operation, Tax Effect of Gain (Loss) from Disposal of Discontinued Operation		153	1,416	(129)
Income Tax Expense (Benefit), Continuing Operations, Discontinued Operations		23,691	22,530	20,994
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		23,224	25,907	19,178
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		(714)	79	(1,435)
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		(75)	(454)	454
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		156	(278)	(528)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		5,107	(3,772)	(250)
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ (3,498)	$ 2,422	$ 3,495

[1] (1) Includes $4.9 million, $4.7 million, and $3.5 million of deferred state income taxes for the years 2020, 2019 and 2018, respectively.