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, 2013		Jun. 30, 2012		Jun. 30, 2013 segment		Jun. 30, 2012		Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of operating segments					3
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 94,146		$ 83,897		$ 234,874		$ 204,811
Intersegment Revenues
Total intersegment revenues	2,220	[1]	1,444	[1]	2,726	[1]	1,957
Operating Income
Total operating income	9,152		10,455		35,702		30,528
Other income, net of other expenses	24		153		312		349
Interest	2,016		2,241		4,088		4,532
Income Before Income Taxes	7,160		8,367		31,926		26,345
Income taxes	2,804		3,307		12,701		10,558
Net Income	4,356		5,060		19,225		15,787		28,863
Identifiable Assets
Total identifiable assets	779,658				779,658				733,746
Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	54,975		55,253		136,279		127,271
Intersegment Revenues
Total intersegment revenues	241	[1]	300	[1]	504	[1]	578	[1]
Operating Income
Total operating income	8,619		10,505		25,925		25,303
Identifiable Assets
Total identifiable assets	658,171				658,171				615,438
Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	34,273		24,253		89,264		69,140
Intersegment Revenues
Total intersegment revenues	1,752	[1]	923	[1]	1,752	[1]	923	[1]
Operating Income
Total operating income	447		(401)		9,816		4,753
Identifiable Assets
Total identifiable assets	92,838				92,838				79,287
Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	4,898		4,391		9,331		8,400
Intersegment Revenues
Total intersegment revenues	227	[1]	221	[1]	470	[1]	456	[1]
Identifiable Assets
Total identifiable assets	28,649				28,649				39,021
Other and eliminations [Member]
Operating Income
Total operating income	$ 86		$ 351		$ (39)		$ 472

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