Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 107,335	$ 101,419	$ 409,604	$ 351,160
Operating Income
Total operating income		20,113	17,406	94,288	77,518
Other expense, net		339	(40)	2,180	2,997
Interest		4,975	4,584	15,134	15,452
Income Before Income Taxes		15,477	12,782	81,334	65,063
Income Taxes on Continuing Operations		2,993	3,502	20,563	16,082
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		12,484	9,280	60,771	48,981
Net Income		12,475	9,261	60,754	49,146
Identifiable Assets
Total identifiable assets		2,021,828		2,021,828		$ 1,932,487
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		(9)	(19)	(17)	165
Regulated Energy [Member]
Operating Income
Total operating income		23,538	20,482	79,210	66,376
Identifiable Assets
Total identifiable assets		1,592,950		1,592,950		1,547,619
Unregulated Energy [Member]
Operating Income
Total operating income		(2,883)	(3,092)	15,777	11,050
Identifiable Assets
Total identifiable assets		381,642		381,642		347,665
Other [Member]
Identifiable Assets
Total identifiable assets		47,236		47,236		$ 37,203
Other and eliminations [Member]
Operating Income
Total operating income		(542)	16	(699)	92
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		107,335	101,419	409,604	351,160
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		79,892	82,297	280,987	257,810
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		27,443	19,122	128,617	93,350
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,302	4,188	14,937	12,937
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	504	465	1,516	1,425
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,666	3,592	13,024	11,116
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	132	131	397	396
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		12,484	9,280	60,771	48,981
Net Income		$ 12,475	$ 9,261	$ 60,754	$ 49,146

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