Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2023 USD ($)		Sep. 30, 2022 USD ($)		Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments					2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 131,547		$ 131,053		$ 485,269	$ 493,403
Operating Income
Total operating income	20,228		18,648		103,488	99,981
Other expense, net	(72)		957		1,036	4,454
Interest	7,076		6,240		21,272	17,404
Income Before Income Taxes	13,080		13,365		83,252	87,031
Income Taxes	3,673		3,703		21,368	23,385
Net Income	9,407		9,662		61,884	63,646
Identifiable Assets
Total identifiable assets	2,240,746				2,240,746		$ 2,215,037
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	5,878		7,886		19,575	20,592
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	131,547		131,053		485,269	493,403
Regulated Energy [Member]
Operating Income
Total operating income	24,912		23,663		91,828	84,202
Identifiable Assets
Total identifiable assets	1,737,163				1,737,163		1,716,255
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	500	[1]	710	[1]	1,632	4,905
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	101,911		90,270		344,190	306,159
Unregulated Energy [Member]
Operating Income
Total operating income	(4,723)		(5,056)		11,529	15,557
Identifiable Assets
Total identifiable assets	451,064				451,064		463,239
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	5,333	[1]	7,131	[1]	17,807	14,950
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	29,636		40,783		141,079	187,244
Other [Member]
Identifiable Assets
Total identifiable assets	52,519				52,519		$ 35,543
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	45	[1]	45	[1]	136	737
Other and eliminations [Member]
Operating Income
Total operating income	39		41		131	222
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	9,407		9,662		61,884	63,646
Net Income	$ 9,407		$ 9,662		$ 61,884	$ 63,646

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