Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments						2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 130,903		$ 136,664		$ 358,519	$ 376,020
Operating Income
Total operating income		17,436		13,248		61,429	53,654
Other expense, net		(316)		(262)		(361)	(194)
Interest		5,655		3,881		11,365	7,545
Income Before Income Taxes		11,465		9,105		49,703	45,915
Income taxes		3,161		2,718		12,735	12,674
Net Income		8,304	$ 28,664	6,387	$ 26,854	36,968	33,241
Identifiable Assets
Total identifiable assets		1,681,042				1,681,042		$ 1,693,671
Regulated Energy [Member]
Operating Income
Total operating income		18,752		14,304		47,769	41,015
Identifiable Assets
Total identifiable assets		1,349,422				1,349,422		1,345,805
Unregulated Energy [Member]
Operating Income
Total operating income		(1,348)		490		13,628	14,174
Identifiable Assets
Total identifiable assets		284,888				284,888		306,045
Other [Member]
Identifiable Assets
Total identifiable assets		46,732				46,732		$ 41,821
Other and eliminations [Member]
Operating Income
Total operating income		32		(1,546)		32	(1,535)
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		130,903		136,664		358,519	376,020
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		70,719		67,731		171,458	173,685
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		60,184		68,933		187,061	202,335
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	9,536		10,379		23,773	25,976
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,684		2,773		5,563	6,212
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	6,720		7,412		17,946	19,377
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	$ 132		$ 194		$ 264	$ 387

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