Segment Information - Schedule of Segment Reporting Information by Segment (Detail)		3 Months Ended
		Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 152,710,000	$ 160,464,000
Operating Income
Total operating income		42,113,000	44,124,000
Other expense, net		3,318,000	(57,000)
Interest		5,814,000	5,628,000
Income Before Income Taxes		39,617,000	38,439,000
Income Taxes on Continuing Operations		10,591,000	9,625,000
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		29,026,000	28,814,000
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		(96,000)	(150,000)
Net Income		28,930,000	28,664,000
Identifiable Assets
Total identifiable assets		1,788,219,000		$ 1,783,198,000
Regulated Energy [Member]
Operating Income
Total operating income		27,888,000	29,741,000
Identifiable Assets
Total identifiable assets		1,448,114,000		1,434,066,000
Unregulated Energy [Member]
Operating Income
Total operating income		13,841,000	15,258,000
Identifiable Assets
Total identifiable assets		295,470,000		296,810,000
Other [Member]
Identifiable Assets
Total identifiable assets		44,635,000		52,322,000
Other and eliminations [Member]
Operating Income
Total operating income		384,000	(875,000)
Continuing Operations [Member]
Identifiable Assets
Total identifiable assets		1,788,219,000		$ 1,783,198,000
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		152,710,000	160,464,000
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		102,494,000	103,071,000
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		50,216,000	57,393,000
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,408,000	4,366,000
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	461,000	547,000
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,815,000	3,687,000
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	132,000	132,000
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		29,026,000	28,814,000
Net Income		$ 28,930,000	$ 28,664,000

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