Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended
		Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 191,187	$ 152,690
Operating Income
Total operating income		51,597	42,134
Other expense, net		385	3,319
Interest		5,105	5,814
Income Before Income Taxes		46,877	39,639
Income Taxes on Continuing Operations		12,405	10,598
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		34,472	29,041
Net Income		34,466	28,930
Identifiable Assets
Total identifiable assets		1,953,950		$ 1,932,487
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		(6)	(111)
Regulated Energy [Member]
Operating Income
Total operating income		32,864	27,888
Identifiable Assets
Total identifiable assets		1,559,260		1,547,619
Unregulated Energy [Member]
Operating Income
Total operating income		19,105	13,862
Identifiable Assets
Total identifiable assets		352,271		347,665
Other [Member]
Identifiable Assets
Total identifiable assets		42,419		$ 37,203
Other and eliminations [Member]
Operating Income
Total operating income		(372)	384
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		191,187	152,690
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		120,721	102,469
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		70,466	50,221
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,901	4,409
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	476	486
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,293	3,791
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	132	132
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		34,472	29,041
Net Income		$ 34,466	$ 28,930

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