Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended
		Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 227,616	$ 239,356
Operating Income
Total operating income		43,993	40,406
Other expense, net		(45)	68
Interest		5,710	3,664
Income Before Income Taxes		38,238	36,810
Income taxes		9,574	9,955
Net Income		28,664	26,855
Identifiable Assets
Total identifiable assets		1,682,797		$ 1,693,671
Regulated Energy [Member]
Operating Income
Total operating income		29,741	26,711
Identifiable Assets
Total identifiable assets		1,343,061		1,345,805
Unregulated Energy [Member]
Operating Income
Total operating income		15,127	13,684
Identifiable Assets
Total identifiable assets		296,836		306,045
Other [Member]
Identifiable Assets
Total identifiable assets		42,900		$ 41,821
Other and eliminations [Member]
Operating Income
Total operating income		(875)	11
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		227,616	239,356
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		100,739	105,954
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		126,877	133,402
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	14,236	15,598
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,879	3,439
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	11,225	11,965
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	$ 132	$ 194

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