Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 137,038	$ 101,419	$ 97,051	$ 152,690	$ 131,974	$ 92,626	$ 94,542	$ 160,464	$ 488,198	$ 479,605	$ 490,316
Intersegment Revenues (1)
Total intersegment revenues		137,038	101,419	97,051	152,690	131,974	92,626	94,542	160,464	488,198	479,605	490,316
Operating Income
Operating Income		35,206	17,406	17,977	42,134	29,641	14,357	18,165	44,122	112,723	106,285	94,844
Other income										3,222	(1,847)	(607)
Interest charges										21,765	22,224	16,146
Income Before Income taxes										94,180	82,214	78,091
Income Taxes on Continuing Operations										23,538	21,114	21,123
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		21,661	9,280	10,661	29,041	17,123	6,251	8,914	28,811	70,642	61,100	56,968
Net Income		22,352	9,261	10,956	28,930	22,564	5,621	8,304	28,663	71,498	65,153	56,580
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		691	$ (19)	$ 125	$ (111)	39	$ (630)	$ (610)	$ (148)	686	(1,349)	(388)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax						5,402				170	5,402
Depreciation and Amortization
Total depreciation and amortization										58,117	45,424	40,220
Identifiable Assets
Total identifiable assets		1,932,487				1,783,198				1,932,487	1,783,198
Regulated Energy
Operating Income
Operating Income										92,124	86,584	79,215
Depreciation and Amortization
Total depreciation and amortization										46,079	35,227	31,876
Payments to Acquire Productive Assets										147,100	130,604	235,912
Unregulated Energy
Operating Income
Operating Income										20,664	19,938	17,125
Depreciation and Amortization
Total depreciation and amortization										11,988	10,130	8,263
Payments to Acquire Productive Assets										46,295	60,034	38,585
Other
Operating Income
Operating Income										(65)	(237)	(1,496)
Depreciation and Amortization
Payments to Acquire Productive Assets										2,480	8,348	8,364
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization										50	67	81
Total for Segments [Member]
Depreciation and Amortization
Payments to Acquire Productive Assets										195,875	198,986	282,861
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										488,198	479,605	490,316
Intersegment Revenues (1)
Total intersegment revenues										488,198	479,605	490,316
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										350,853	340,857	343,313
Intersegment Revenues (1)
Total intersegment revenues										350,853	340,857	343,313
Identifiable Assets
Total identifiable assets		1,547,619				1,434,066				1,547,619	1,434,066
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										137,345	138,748	147,003
Intersegment Revenues (1)
Total intersegment revenues										137,345	138,748	147,003
Identifiable Assets
Total identifiable assets		347,665				296,810				347,665	296,810
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									17,602	18,081	17,522
Intersegment Revenues (1)
Total intersegment revenues	[1]									17,602	18,081	17,522
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									1,893	2,149	1,968
Intersegment Revenues (1)
Total intersegment revenues	[1]									1,893	2,149	1,968
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									15,181	15,403	14,902
Intersegment Revenues (1)
Total intersegment revenues	[1]									15,181	15,403	14,902
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									528	529	652
Intersegment Revenues (1)
Total intersegment revenues	[1]									528	529	$ 652
Other And Intersegment Eliminations
Identifiable Assets
Total identifiable assets		$ 37,203				$ 52,322				$ 37,203	$ 52,322

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