Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2023 USD ($)		Jun. 30, 2022 USD ($)		Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments					2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 135,593		$ 139,470		$ 353,722	$ 362,350
Operating Income
Total operating income	28,346		26,469		83,261	81,334
Other expense, net	831		2,584		1,107	3,498
Interest	6,964		5,825		14,196	11,164
Income Before Income Taxes	22,213		23,228		70,172	73,668
Income Taxes	6,080		6,177		17,695	19,683
Net Income	16,133		17,051		52,477	53,985
Identifiable Assets
Total identifiable assets	2,204,490				2,204,490		$ 2,215,037
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	6,345		6,271		13,696	12,706
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	135,593		139,470		353,722	362,350
Regulated Energy [Member]
Operating Income
Total operating income	29,291		25,841		66,916	60,539
Identifiable Assets
Total identifiable assets	1,707,605				1,707,605		1,716,255
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	484	[1]	363	[1]	1,132	4,196
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	100,657		91,830		242,279	215,888
Unregulated Energy [Member]
Operating Income
Total operating income	(993)		560		16,252	20,613
Identifiable Assets
Total identifiable assets	448,217				448,217		463,239
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	5,816	[1]	5,823	[1]	12,473	7,819
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	34,936		47,640		111,443	146,462
Other [Member]
Identifiable Assets
Total identifiable assets	48,668				48,668		$ 35,543
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	45	[1]	85	[1]	91	691
Other and eliminations [Member]
Operating Income
Total operating income	48		68		93	182
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	16,133		17,051		52,477	53,985
Net Income	$ 16,133		$ 17,051		$ 52,477	$ 53,985

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