Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended
		Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 222,880	$ 191,187
Operating Income
Total operating income		54,865	51,597
Other expense, net		913	375
Interest		5,339	5,105
Income Before Income Taxes		50,439	46,867
Income Taxes		13,506	12,401
Net Income		36,933	34,466
Identifiable Assets
Total identifiable assets		2,109,629		$ 2,114,869
Regulated Energy [Member]
Operating Income
Total operating income		34,681	32,637
Identifiable Assets
Total identifiable assets		1,620,209		1,629,191
Unregulated Energy [Member]
Operating Income
Total operating income		20,046	18,983
Identifiable Assets
Total identifiable assets		440,936		439,114
Other [Member]
Identifiable Assets
Total identifiable assets		48,484		$ 46,564
Other and eliminations [Member]
Operating Income
Total operating income		138	(23)
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		222,880	191,187
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		124,058	120,721
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		98,822	70,466
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	6,435	4,901
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,833	476
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	1,996	4,293
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	606	132
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		36,933	34,466
Net Income		$ 36,933	$ 34,466

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