Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 100,497		$ 94,146		$ 286,834		$ 234,874
Intersegment Revenues
Total operating revenues, unaffiliated customers	100,497		94,146		286,834		234,874
Operating Income
Total operating income	10,457		9,152		42,079		35,702
Other income, net of other expenses	405		24		413		312
Interest	2,303		2,016		4,459		4,088
Income Before Income Taxes	8,559		7,160		38,033		31,926
Income taxes	3,425		2,804		15,218		12,701
Net Income	5,134		4,356		22,815		19,225		32,787
Identifiable Assets
Total identifiable assets	821,085				821,085				837,522
Regulated Energy [Member]
Operating Income
Total operating income	10,711		8,619		31,802		25,925
Identifiable Assets
Total identifiable assets	716,126				716,126				708,950
Unregulated Energy [Member]
Operating Income
Total operating income	(43)		447		10,815		9,816
Identifiable Assets
Total identifiable assets	77,800				77,800				100,585
Other [Member]
Identifiable Assets
Total identifiable assets	27,159				27,159				27,987
Other and eliminations [Member]
Operating Income
Total operating income	(211)		86		(538)		(39)
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	100,497		94,146		286,834		234,874
Intersegment Revenues
Total operating revenues, unaffiliated customers	100,497		94,146		286,834		234,874
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	61,348		54,975		163,222		136,279
Intersegment Revenues
Total operating revenues, unaffiliated customers	61,348		54,975		163,222		136,279
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	34,299		34,273		114,173		89,264
Intersegment Revenues
Total operating revenues, unaffiliated customers	34,299		34,273		114,173		89,264
Operating Revenues, Unaffiliated Customers [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	4,850		4,898		9,439		9,331
Intersegment Revenues
Total operating revenues, unaffiliated customers	4,850		4,898		9,439		9,331
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	568	[1]	2,220	[1]	1,213		2,726
Intersegment Revenues
Total operating revenues, unaffiliated customers	568	[1]	2,220	[1]	1,213		2,726
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	298	[1]	241	[1]	590	[1]	504	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	298	[1]	241	[1]	590	[1]	504	[1]
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	22	[1]	1,752	[1]	121	[1]	1,752	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	22	[1]	1,752	[1]	121	[1]	1,752	[1]
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	248	[1]	227	[1]	502	[1]	470	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	$ 248	[1]	$ 227	[1]	$ 502	[1]	$ 470	[1]

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