Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 131,973	$ 92,626	$ 94,541	$ 160,464	$ 134,214		$ 93,400	$ 93,872	$ 168,831	$ 479,604	$ 490,316		$ 449,646
Intersegment Revenues (1)
Total intersegment revenues		131,973	92,626	94,541	160,464	134,214		93,400	93,872	168,831	479,604	490,316		449,646
Operating Income
Operating Income		29,642	14,358	18,164	44,123	28,873		12,879	12,238	40,853	106,287	94,843		89,730
Other income											(1,830)	(603)		(2,204)
Interest charges											22,224	16,146		12,530
Income Before Income taxes											82,233	78,094		74,996
Income Taxes on Continuing Operations											21,091	21,232		14,670
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		17,169	6,246	8,913	28,814	17,796		6,090	5,705	27,271	61,142	56,862		60,326
Net Income		22,562	5,622	8,304	28,665	17,801		5,538	6,385	26,856	65,153	56,580		58,124
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		(9)	(624)	(609)	(149)	5		$ (552)	$ 680	$ (415)	(1,391)	(282)		(2,202)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax		5,402	$ 0	$ 0	$ 0						5,402
Depreciation and Amortization
Total depreciation and amortization											45,423	40,220		36,386
Identifiable Assets
Total identifiable assets		1,783,198				1,693,671					1,783,198	1,693,671
Regulated Energy
Operating Income
Operating Income											86,584	79,215		74,584
Depreciation and Amortization
Total depreciation and amortization											35,227	31,876		28,554
Payments to Acquire Productive Assets											130,604	235,912		159,011
Identifiable Assets
Total identifiable assets		1,434,066				1,345,805					1,434,066	1,345,805
Unregulated Energy
Operating Income
Operating Income											19,939	17,124		14,941
Depreciation and Amortization
Total depreciation and amortization											10,129	8,263		7,741
Payments to Acquire Productive Assets											60,034	38,585		14,424
Identifiable Assets
Total identifiable assets		296,810				245,702	[1]				296,810	245,702	[1]
Other
Operating Income
Operating Income											(236)	(1,496)		205
Depreciation and Amortization
Payments to Acquire Productive Assets											8,348	8,364		5,902
Identifiable Assets
Total identifiable assets		$ 52,322				41,821					52,322	41,821
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization											67	81		91
Total for Segments [Member]
Depreciation and Amortization
Payments to Acquire Productive Assets											198,986	282,861		179,337
Continuing Operations [Member]
Identifiable Assets
Total identifiable assets	[1]					$ 1,633,328						1,633,328
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues											479,604	490,316		449,646
Intersegment Revenues (1)
Total intersegment revenues											479,604	490,316		449,646
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues											340,857	343,313		323,972
Intersegment Revenues (1)
Total intersegment revenues											340,857	343,313		323,972
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues											138,747	147,003		125,674
Intersegment Revenues (1)
Total intersegment revenues											138,747	147,003		125,674
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[2]										18,080	17,522		17,514
Intersegment Revenues (1)
Total intersegment revenues	[2]										18,080	17,522		17,514
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[2]										2,149	1,968		2,338
Intersegment Revenues (1)
Total intersegment revenues	[2]										2,149	1,968		2,338
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[2]										15,403	14,902		14,402
Intersegment Revenues (1)
Total intersegment revenues	[2]										15,403	14,902		14,402
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[2]										528	652		774
Intersegment Revenues (1)
Total intersegment revenues	[2]										$ 528	$ 652		$ 774

[1] ⁽¹⁾ 2018 balance excludes assets held for sale related to the sale of assets and contracts for PESCO.

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