Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 101,419	$ 92,626	$ 351,160	$ 347,630
Operating Income
Total operating income		17,406	14,357	77,518	76,645
Other expense, net		(40)	(351)	2,997	(731)
Interest		4,584	5,403	15,452	16,583
Income Before Income Taxes		12,782	8,603	65,063	59,331
Income Taxes on Continuing Operations		3,502	2,352	16,082	15,354
Net Income		9,261	5,621	49,146	42,589
Identifiable Assets
Total identifiable assets		1,888,088		1,888,088		$ 1,783,198
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		(19)	(630)	165	(1,388)
Regulated Energy [Member]
Operating Income
Total operating income		20,482	17,540	66,376	65,310
Identifiable Assets
Total identifiable assets		1,530,689		1,530,689		1,434,066
Unregulated Energy [Member]
Operating Income
Total operating income		(3,092)	(3,169)	11,050	11,317
Identifiable Assets
Total identifiable assets		305,536		305,536		296,810
Other [Member]
Identifiable Assets
Total identifiable assets		51,863		51,863		52,322
Other and eliminations [Member]
Operating Income
Total operating income		16	(14)	92	18
Continuing Operations [Member]
Identifiable Assets
Total identifiable assets		1,888,088		1,888,088		$ 1,783,198
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		101,419	92,626	351,160	347,630
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		82,297	74,027	257,810	249,978
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		19,122	18,599	93,350	97,652
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,188	4,367	12,937	13,351
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	465	553	1,425	1,623
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,592	3,681	11,116	11,333
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	131	133	396	395
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		9,280	6,251	48,981	43,977
Net Income		$ 9,261	$ 5,621	$ 49,146	$ 42,589

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