Segment Information - Schedule of Segment Reporting Information by Segment (Detail)		3 Months Ended		9 Months Ended
		Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 92,626,000	$ 93,400,000	$ 347,630,000	$ 356,102,000
Operating Income
Total operating income		14,358,000	12,879,000	76,644,000	65,968,000
Other expense, net		(350,000)	(4,000)	(729,000)	(168,000)
Interest		5,403,000	4,357,000	16,583,000	11,764,000
Income Before Income Taxes		8,605,000	8,518,000	59,332,000	54,036,000
Income Taxes on Continuing Operations		2,360,000	2,428,000	15,355,000	14,918,000
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		6,245,000	6,090,000	43,977,000	39,118,000
Net Income		5,621,000	5,538,000	42,589,000	38,779,000
Identifiable Assets
Total identifiable assets		1,714,240,000		1,714,240,000		$ 1,693,671,000
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax		(624,000)	(552,000)	(1,388,000)	(339,000)
Regulated Energy [Member]
Operating Income
Total operating income		17,540,000	15,915,000	65,310,000	56,930,000
Identifiable Assets
Total identifiable assets		1,384,057,000		1,384,057,000		1,345,805,000
Unregulated Energy [Member]
Operating Income
Total operating income		(3,168,000)	(3,090,000)	11,316,000	10,519,000
Identifiable Assets
Total identifiable assets	[1]	250,849,000		250,849,000		245,702,000
Other [Member]
Identifiable Assets
Total identifiable assets		51,000,000		51,000,000		41,821,000
Other and eliminations [Member]
Operating Income
Total operating income		(14,000)	54,000	18,000	(1,481,000)
Continuing Operations [Member]
Identifiable Assets
Total identifiable assets		1,685,906,000		1,685,906,000		$ 1,633,328,000
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		92,626,000	93,400,000	347,630,000	356,102,000
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		74,027,000	72,187,000	249,978,000	251,259,000
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		18,599,000	21,213,000	97,652,000	104,843,000
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[2]	4,367,000	3,928,000	13,351,000	12,864,000
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[2]	553,000	583,000	1,623,000	1,408,000
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[2]	3,682,000	3,210,000	11,332,000	10,935,000
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[2]	132,000	135,000	396,000	521,000
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		6,245,000	6,090,000	43,977,000	39,118,000
Net Income		$ 5,621,000	$ 5,538,000	$ 42,589,000	$ 38,779,000

[1] ⁽¹⁾ 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 operating revenues.