Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended				9 Months Ended		12 Months Ended
		Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 91,913		$ 91,619		$ 354,676	$ 378,454
Intersegment Revenues
Total operating revenues, unaffiliated customers		91,913		91,619		354,676	378,454
Operating Income
Total operating income		10,909		7,792		61,587	49,872
Other income, net of other expenses		36		(32)		(3)	380
Interest		2,492		2,495		7,425	6,954
Income Before Income Taxes		8,453		5,265		54,159	43,298
Income taxes		3,334		2,085		21,638	17,303
Net Income		5,119		3,180		32,521	25,995	$ 36,092
Identifiable Assets
Total identifiable assets		1,008,442				1,008,442		904,469
Regulated Energy [Member]
Operating Income
Total operating income		11,828		9,202		47,616	41,004
Identifiable Assets
Total identifiable assets		824,330				824,330		796,021
Unregulated Energy [Member]
Operating Income
Total operating income		(1,022)		(1,972)		13,666	8,843
Identifiable Assets
Total identifiable assets		156,838				156,838		84,732
Other [Member]
Identifiable Assets
Total identifiable assets		27,274				27,274		$ 23,716
Other and eliminations [Member]
Operating Income
Total operating income		103		562		305	25
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		91,913		91,619		354,676	378,454
Intersegment Revenues
Total operating revenues, unaffiliated customers		91,913		91,619		354,676	378,454
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		63,526		59,086		234,608	222,308
Intersegment Revenues
Total operating revenues, unaffiliated customers		63,526		59,086		234,608	222,308
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		28,387		27,041		120,068	141,215
Intersegment Revenues
Total operating revenues, unaffiliated customers		28,387		27,041		120,068	141,215
Operating Revenues, Unaffiliated Customers [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		0		5,492		0	14,931
Intersegment Revenues
Total operating revenues, unaffiliated customers		0		5,492		0	14,931
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		1,712	[1]	558	[1]	4,585	1,770
Intersegment Revenues
Total operating revenues, unaffiliated customers		1,712	[1]	558	[1]	4,585	1,770
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	270		270		830	860
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	270		270		830	860
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	1,222		30		3,095	150
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	1,222		30		3,095	150
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	220		258		660	760
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 220		$ 258		$ 660	$ 760

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