Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 97,051	$ 94,542	$ 249,741	$ 255,005
Operating Income
Total operating income		17,977	18,165	60,111	62,287
Other expense, net		(279)	(320)	3,039	(380)
Interest		5,054	5,552	10,868	11,180
Income Before Income Taxes		12,644	12,293	52,282	50,727
Income Taxes on Continuing Operations		1,983	3,379	12,580	13,002
Net Income		10,956	8,304	39,886	36,968
Identifiable Assets
Total identifiable assets		1,822,175		1,822,175		$ 1,783,198
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		295	(610)	184	(757)
Regulated Energy [Member]
Operating Income
Total operating income		18,006	18,028	45,894	47,769
Identifiable Assets
Total identifiable assets		1,477,616		1,477,616		1,434,066
Unregulated Energy [Member]
Operating Income
Total operating income		281	(771)	14,142	14,486
Identifiable Assets
Total identifiable assets		296,140		296,140		296,810
Other [Member]
Identifiable Assets
Total identifiable assets		48,419		48,419		52,322
Other and eliminations [Member]
Operating Income
Total operating income		(310)	908	75	32
Continuing Operations [Member]
Identifiable Assets
Total identifiable assets		1,822,175		1,822,175		$ 1,783,198
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		97,051	94,542	249,741	255,005
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		73,043	72,880	175,536	175,951
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		24,008	21,662	74,205	79,054
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,340	4,618	8,749	8,984
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	475	523	937	1,070
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,733	3,963	7,548	7,650
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	132	132	264	264
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		10,661	8,914	39,702	37,725
Net Income		$ 10,956	$ 8,304	$ 39,886	$ 36,968

[1] All significant intersegment revenues are billed at market rates and have been eliminated from consolidated operating revenues.