Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 186,337		$ 140,729
Intersegment Revenues
Total operating revenues, unaffiliated customers	186,337		140,729
Operating Income
Total operating income	31,623		26,550
Other income, net of other expenses	6		289
Interest	2,155		2,072
Income Before Income Taxes	29,474		24,767
Income taxes	11,793		9,898
Net Income	17,681		14,869		32,787
Identifiable Assets
Total identifiable assets	842,561				837,522
Regulated Energy [Member]
Operating Income
Total operating income	21,091		17,306
Identifiable Assets
Total identifiable assets	715,062				708,950
Unregulated Energy [Member]
Operating Income
Total operating income	10,858		9,369
Identifiable Assets
Total identifiable assets	103,658				100,585
Other [Member]
Identifiable Assets
Total identifiable assets	23,841				27,987
Other and eliminations [Member]
Operating Income
Total operating income	(326)		(125)
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	186,337		140,729
Intersegment Revenues
Total operating revenues, unaffiliated customers	186,337		140,729
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	101,874		81,304
Intersegment Revenues
Total operating revenues, unaffiliated customers	101,874		81,304
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	79,874		54,991
Intersegment Revenues
Total operating revenues, unaffiliated customers	79,874		54,991
Operating Revenues, Unaffiliated Customers [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	4,589		4,434
Intersegment Revenues
Total operating revenues, unaffiliated customers	4,589		4,434
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	644	[1]	506	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	644	[1]	506	[1]
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	292	[1]	263	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	292	[1]	263	[1]
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	99	[1]	0	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	99	[1]	0	[1]
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	253	[1]	243	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	$ 253	[1]	$ 243	[1]

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