Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 680,704	$ 569,968	$ 488,198
Intersegment Revenues (1)
Total intersegment revenues		680,704	569,968	488,198
Operating Income
Operating Income		142,933	131,112	112,723
Other income		5,051	1,721	3,222
Interest charges		24,356	20,135	21,765
Income Before Income taxes		123,628	112,698	94,180
Income Taxes on Continuing Operations		33,832	29,231	23,538
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		89,796	83,467	70,642
Net Income		89,796	83,466	71,498
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		0	(1)	686
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax		0	0	170
Depreciation and Amortization
Total depreciation and amortization		68,973	62,661	58,117
Identifiable Assets
Total identifiable assets		2,215,037	2,114,869
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		0	(1)	686
Regulated Energy
Operating Income
Operating Income		115,317	106,174	92,124
Depreciation and Amortization
Total depreciation and amortization		52,707	48,748	46,079
Payments to Acquire Productive Assets		97,554	139,733	147,100
Unregulated Energy
Operating Income
Operating Income		27,350	24,427	20,664
Depreciation and Amortization
Total depreciation and amortization		16,257	13,869	11,988
Payments to Acquire Productive Assets		40,773	81,651	46,295
Other
Operating Income
Operating Income		266	511	(65)
Depreciation and Amortization
Payments to Acquire Productive Assets		2,355	6,425	2,480
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization		9	44	50
Total for Segments [Member]
Depreciation and Amortization
Payments to Acquire Productive Assets		140,682	227,809	195,875
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		680,704	569,968	488,198
Intersegment Revenues (1)
Total intersegment revenues		680,704	569,968	488,198
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		422,894	381,879	350,853
Intersegment Revenues (1)
Total intersegment revenues		422,894	381,879	350,853
Identifiable Assets
Total identifiable assets		1,716,255	1,629,191
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		257,810	188,089	137,345
Intersegment Revenues (1)
Total intersegment revenues		257,810	188,089	137,345
Identifiable Assets
Total identifiable assets		463,239	439,114
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]	29,778	21,348	17,602
Intersegment Revenues (1)
Total intersegment revenues	[1]	29,778	21,348	17,602
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		6,530	2,041	1,893
Intersegment Revenues (1)
Total intersegment revenues		6,530	2,041	1,893
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		22,940	18,780	15,181
Intersegment Revenues (1)
Total intersegment revenues		22,940	18,780	15,181
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		308	527	528
Intersegment Revenues (1)
Total intersegment revenues		308	527	$ 528
Other And Intersegment Eliminations
Identifiable Assets
Total identifiable assets		$ 35,543	$ 46,564

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