Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 160,138	$ 131,547	$ 572,154	$ 485,269
Operating Income
Total operating income		40,918	20,228	161,293	103,488
Other expense, net		400	(72)	1,705	1,036
Interest		17,022	7,076	50,861	21,272
Income Before Income Taxes		24,296	13,080	112,137	83,252
Income taxes		6,789	3,673	30,191	21,368
Net Income		17,507	9,407	81,946	61,884
Identifiable Assets
Total identifiable assets		3,467,192		3,467,192		$ 3,304,704
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		6,107	5,878	17,755	19,575
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		160,138	131,547	572,154	485,269
Regulated Energy [Member]
Operating Income
Total operating income		43,935	24,912	142,549	91,828
Identifiable Assets
Total identifiable assets		2,967,280		2,967,280		2,781,581
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	476	500	4,600	1,632
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		130,157	101,911	425,084	344,190
Unregulated Energy [Member]
Operating Income
Total operating income		(3,061)	(4,723)	18,606	11,529
Identifiable Assets
Total identifiable assets		448,024		448,024		477,402
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,586	5,333	13,019	17,807
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		29,981	29,636	147,070	141,079
Other [Member]
Identifiable Assets
Total identifiable assets		51,888		51,888		$ 45,721
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	45	45	136	136
Other and eliminations [Member]
Operating Income
Total operating income		44	39	138	131
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		17,507	9,407	81,946	61,884
Net Income		$ 17,507	$ 9,407	$ 81,946	$ 61,884

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