Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 166,272	$ 135,593	$ 412,016	$ 353,722
Operating Income
Total operating income		40,790	28,346	120,375	83,261
Other expense, net		1,110	831	1,305	1,107
Interest		16,813	6,964	33,839	14,196
Income Before Income Taxes		25,087	22,213	87,841	70,172
Income taxes		6,816	6,080	23,402	17,695
Net Income		18,271	16,133	64,439	52,477
Identifiable Assets
Total identifiable assets		3,380,753		3,380,753		$ 3,304,704
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		5,818	6,345	11,648	13,696
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		166,272	135,593	412,016	353,722
Regulated Energy [Member]
Operating Income
Total operating income		40,505	29,291	98,614	66,916
Identifiable Assets
Total identifiable assets		2,882,798		2,882,798		2,781,581
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,625	484	4,124	1,132
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		127,000	100,657	294,927	242,279
Unregulated Energy [Member]
Operating Income
Total operating income		238	(993)	21,667	16,252
Identifiable Assets
Total identifiable assets		447,913		447,913		477,402
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,147	5,816	7,433	12,473
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		39,272	34,936	117,089	111,443
Other [Member]
Identifiable Assets
Total identifiable assets		50,042		50,042		$ 45,721
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	46	45	91	91
Other and eliminations [Member]
Operating Income
Total operating income		47	48	94	93
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		18,271	16,133	64,439	52,477
Net Income		$ 18,271	$ 16,133	$ 64,439	$ 52,477

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