Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 108,348	$ 91,913	$ 356,986	$ 354,676
Intersegment Revenues
Total operating revenues, unaffiliated customers		108,348	91,913	356,986	354,676
Operating Income
Total operating income		10,156	10,909	62,277	61,587
Other income, net of other expenses		(28)	36	(68)	(3)
Interest		2,722	2,492	7,996	7,425
Income Before Income Taxes		7,406	8,453	54,213	54,159
Income taxes		2,990	3,334	21,401	21,638
Net Income		4,416	5,119	32,812	32,521	$ 41,140
Identifiable Assets
Total identifiable assets		1,140,510		1,140,510		1,067,421
Regulated Energy [Member]
Operating Income
Total operating income		13,115	11,828	52,660	47,616
Identifiable Assets
Total identifiable assets		921,682		921,682		872,065
Unregulated Energy [Member]
Operating Income
Total operating income		(3,080)	(1,022)	9,267	13,666
Identifiable Assets
Total identifiable assets		207,083		207,083		171,840
Other [Member]
Identifiable Assets
Total identifiable assets		11,745		11,745		$ 23,516
Other and eliminations [Member]
Operating Income
Total operating income		121	103	350	305
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		108,348	91,913	356,986	354,676
Intersegment Revenues
Total operating revenues, unaffiliated customers		108,348	91,913	356,986	354,676
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		68,899	63,526	224,382	234,608
Intersegment Revenues
Total operating revenues, unaffiliated customers		68,899	63,526	224,382	234,608
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		39,449	28,387	132,604	120,068
Intersegment Revenues
Total operating revenues, unaffiliated customers		39,449	28,387	132,604	120,068
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,953	1,712	6,712	4,585
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	3,953	1,712	6,712	4,585
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	1,120	270	2,248	830
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	1,120	270	2,248	830
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,593	1,222	3,759	3,095
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	2,593	1,222	3,759	3,095
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	240	220	705	660
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 240	$ 220	$ 705	$ 660

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