Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended
		Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 245,744	$ 218,129
Operating Income
Total operating income		79,585	54,915
Other expense, net		195	276
Interest		17,026	7,232
Income Before Income Taxes		62,754	47,959
Income taxes		16,586	11,615
Net Income		46,168	36,344
Identifiable Assets
Total identifiable assets		3,331,884		$ 3,304,704
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		5,830	7,351
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		245,744	218,129
Regulated Energy [Member]
Operating Income
Total operating income		58,109	37,625
Identifiable Assets
Total identifiable assets		2,830,632		2,781,581
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	499	649
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		167,927	141,621
Unregulated Energy [Member]
Operating Income
Total operating income		21,429	17,245
Identifiable Assets
Total identifiable assets		459,825		477,402
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,286	6,657
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		77,817	76,508
Other [Member]
Identifiable Assets
Total identifiable assets		41,427		$ 45,721
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	45	45
Other and eliminations [Member]
Operating Income
Total operating income		47	45
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		46,168	36,344
Net Income		$ 46,168	$ 36,344

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