Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 111,082	$ 97,051	$ 302,268	$ 249,741
Operating Income
Total operating income		22,578	17,977	74,175	60,111
Other expense, net		1,456	(279)	1,841	3,039
Interest		5,054	5,054	10,159	10,868
Income Before Income Taxes		18,980	12,644	65,857	52,282
Income Taxes on Continuing Operations		5,165	1,983	17,570	12,580
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		13,815	10,661	48,287	39,702
Net Income		13,813	10,956	48,279	39,886
Identifiable Assets
Total identifiable assets		1,972,340		1,972,340		$ 1,932,487
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		(2)	295	(8)	184
Regulated Energy [Member]
Operating Income
Total operating income		22,808	18,006	55,673	45,894
Identifiable Assets
Total identifiable assets		1,568,136		1,568,136		1,547,619
Unregulated Energy [Member]
Operating Income
Total operating income		(445)	281	18,660	14,142
Identifiable Assets
Total identifiable assets		359,971		359,971		347,665
Other [Member]
Identifiable Assets
Total identifiable assets		44,233		44,233		$ 37,203
Other and eliminations [Member]
Operating Income
Total operating income		215	(310)	(158)	75
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		111,082	97,051	302,268	249,741
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		80,374	73,044	201,095	175,512
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		30,708	24,007	101,173	74,229
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,734	4,340	9,635	8,749
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	536	474	1,012	961
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,065	3,733	8,358	7,523
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	133	133	265	265
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		13,815	10,661	48,287	39,702
Net Income		$ 13,813	$ 10,956	$ 48,279	$ 39,886

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