Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 126,936	$ 108,348	$ 437,180	$ 356,986
Intersegment Revenues
Total operating revenues, unaffiliated customers		126,936	108,348	437,180	356,986
Operating Income
Total operating income		14,239	10,156	62,580	62,277
Other expense, net		239	(28)	(643)	(68)
Interest		3,321	2,722	9,133	7,996
Income Before Income Taxes		11,157	7,406	52,804	54,213
Income taxes		4,324	2,990	20,781	21,401
Net Income		6,833	4,416	32,023	32,812	$ 44,675
Identifiable Assets
Total identifiable assets		1,346,750		1,346,750		1,229,219
Regulated Energy [Member]
Operating Income
Total operating income		15,168	13,115	51,915	52,660
Identifiable Assets
Total identifiable assets		1,084,961		1,084,961		986,752
Unregulated Energy [Member]
Operating Income
Total operating income		(989)	(3,080)	10,504	9,267
Identifiable Assets
Total identifiable assets		233,785		233,785		226,368
Other [Member]
Identifiable Assets
Total identifiable assets		28,004		28,004		$ 16,099
Other and eliminations [Member]
Operating Income
Total operating income		60	121	161	350
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		126,936	108,348	437,180	356,986
Intersegment Revenues
Total operating revenues, unaffiliated customers		126,936	108,348	437,180	356,986
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		67,257	68,899	232,519	224,382
Intersegment Revenues
Total operating revenues, unaffiliated customers		67,257	68,899	232,519	224,382
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		59,679	39,449	204,661	132,604
Intersegment Revenues
Total operating revenues, unaffiliated customers		59,679	39,449	204,661	132,604
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	7,649	3,953	22,216	6,712
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	7,649	3,953	22,216	6,712
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,446	1,120	5,834	2,248
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	2,446	1,120	5,834	2,248
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,009	2,593	15,801	3,759
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	5,009	2,593	15,801	3,759
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	194	240	581	705
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 194	$ 240	$ 581	$ 705

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