Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 102,342	$ 92,682	$ 248,638	$ 262,763
Intersegment Revenues
Total operating revenues, unaffiliated customers		102,342	92,682	248,638	262,763
Operating Income
Total operating income		15,742	13,170	52,122	50,678
Other income, net of other expenses		(8)	(171)	(42)	(38)
Interest		2,624	2,485	5,274	4,933
Income Before Income Taxes		13,110	10,514	46,806	45,707
Income taxes		5,081	4,220	18,410	18,304
Net Income		8,029	6,294	28,396	27,403	$ 41,140
Identifiable Assets
Total identifiable assets		1,097,047		1,097,047		1,067,421
Regulated Energy [Member]
Operating Income
Total operating income		15,226	13,605	39,545	35,788
Identifiable Assets
Total identifiable assets		892,513		892,513		872,065
Unregulated Energy [Member]
Operating Income
Total operating income		412	(540)	12,347	14,689
Identifiable Assets
Total identifiable assets		192,654		192,654		171,840
Other [Member]
Identifiable Assets
Total identifiable assets		11,880		11,880		$ 23,516
Other and eliminations [Member]
Operating Income
Total operating income		104	105	230	201
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		102,342	92,682	248,638	262,763
Intersegment Revenues
Total operating revenues, unaffiliated customers		102,342	92,682	248,638	262,763
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		66,590	61,790	155,483	171,082
Intersegment Revenues
Total operating revenues, unaffiliated customers		66,590	61,790	155,483	171,082
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		35,752	30,892	93,155	91,681
Intersegment Revenues
Total operating revenues, unaffiliated customers		35,752	30,892	93,155	91,681
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,097	2,156	2,759	2,873
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	2,097	2,156	2,759	2,873
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	805	270	1,128	560
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	805	270	1,128	560
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	1,052	1,666	1,165	1,873
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	1,052	1,666	1,165	1,873
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	240	220	466	440
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 240	$ 220	$ 466	$ 440

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