Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 140,279	$ 126,936	$ 516,299	$ 437,180
Intersegment Revenues
Total operating revenues, unaffiliated customers		140,279	126,936	516,299	437,180
Operating Income
Total operating income		12,036	14,632	65,690	63,792
Other expense, net		(11)	(154)	(204)	(1,855)
Interest		4,430	3,321	11,976	9,133
Income Before Income Taxes		7,595	11,157	53,510	52,804
Income taxes		2,057	4,324	14,731	20,781
Net Income		5,538	6,833	38,779	32,023	$ 58,124
Identifiable Assets
Total identifiable assets		1,505,556		1,505,556		1,414,934
Regulated Energy [Member]
Operating Income
Total operating income		15,915	15,523	56,930	53,004
Identifiable Assets
Total identifiable assets		1,223,537		1,223,537		1,121,673
Unregulated Energy [Member]
Operating Income
Total operating income		(3,933)	(951)	10,241	10,626
Identifiable Assets
Total identifiable assets		244,390		244,390		259,041
Other [Member]
Identifiable Assets
Total identifiable assets		37,629		37,629		$ 34,220
Other and eliminations [Member]
Operating Income
Total operating income		54	60	(1,481)	162
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		140,279	126,936	516,299	437,180
Intersegment Revenues
Total operating revenues, unaffiliated customers		140,279	126,936	516,299	437,180
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		69,697	67,257	243,382	232,519
Intersegment Revenues
Total operating revenues, unaffiliated customers		69,697	67,257	243,382	232,519
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		70,582	59,679	272,917	204,661
Intersegment Revenues
Total operating revenues, unaffiliated customers		70,582	59,679	272,917	204,661
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	8,668	7,649	34,643	22,216
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	8,668	7,649	34,643	22,216
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,073	2,446	9,285	5,834
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	3,073	2,446	9,285	5,834
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,460	5,009	24,837	15,801
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	5,460	5,009	24,837	15,801
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	135	194	521	581
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 135	$ 194	$ 521	$ 581

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