Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended
		Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 218,129	$ 222,880
Operating Income
Total operating income		54,915	54,865
Other expense, net		276	913
Interest		7,232	5,339
Income Before Income Taxes		47,959	50,439
Income Taxes		11,615	13,506
Net Income		36,344	36,933
Identifiable Assets
Total identifiable assets		2,197,551		$ 2,215,037
Regulated Energy [Member]
Operating Income
Total operating income		37,625	34,681
Identifiable Assets
Total identifiable assets		1,713,149		1,716,255
Unregulated Energy [Member]
Operating Income
Total operating income		17,245	20,046
Identifiable Assets
Total identifiable assets		441,261		463,239
Other [Member]
Identifiable Assets
Total identifiable assets		43,141		$ 35,543
Other and eliminations [Member]
Operating Income
Total operating income		45	138
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		218,129	222,880
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		141,621	124,058
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		76,508	98,822
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	7,351	6,435
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	649	3,833
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	6,657	1,996
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	45	606
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		36,344	36,933
Net Income		$ 36,344	$ 36,933

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