Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current Income Tax Expense
Federal		$ (2,271)	$ 48	$ 2,046
State		(492)	581	610
Other		(47)	(47)	(71)
Total current income tax expense (benefit)		(2,810)	582	2,585
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	23,901	20,650	12,085
Income Tax Expense from Continuing Operations		21,091	21,232	14,670
Discontinued Operation, Tax Effect of Gain (Loss) from Disposal of Discontinued Operation		(1,439)	(238)	(361)
Income Tax Expense (Benefit), Continuing Operations, Discontinued Operations		22,530	20,994	14,309
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		25,910	19,189	8,181
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		79	(1,435)	2,002
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		(454)	446	180
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		(278)	(528)	(1,148)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		(3,776)	(183)	193
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ 2,420	$ 3,161	$ 2,677

[1] ⁽¹⁾ Includes $4.7 million, $3.5 million, and $0.9 million of deferred state income taxes for the years 2019, 2018 and 2017, respectively.