Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 170,081		$ 186,337
Intersegment Revenues
Total operating revenues, unaffiliated customers	170,081		186,337
Operating Income
Total operating income	37,508		31,623
Other income, net of other expenses	133		6
Interest	2,448		2,155
Income Before Income Taxes	35,193		29,474
Income taxes	14,084		11,793
Net Income	21,109		17,681		36,092
Identifiable Assets
Total identifiable assets	913,707				904,469
Regulated Energy [Member]
Operating Income
Total operating income	22,182		21,091
Identifiable Assets
Total identifiable assets	788,600				796,021
Unregulated Energy [Member]
Operating Income
Total operating income	15,229		10,858
Identifiable Assets
Total identifiable assets	89,950				84,732
Other [Member]
Identifiable Assets
Total identifiable assets	35,157				23,716
Other and eliminations [Member]
Operating Income
Total operating income	97		(326)
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	170,081		186,337
Intersegment Revenues
Total operating revenues, unaffiliated customers	170,081		186,337
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	109,292		101,874
Intersegment Revenues
Total operating revenues, unaffiliated customers	109,292		101,874
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	60,789		79,874
Intersegment Revenues
Total operating revenues, unaffiliated customers	60,789		79,874
Operating Revenues, Unaffiliated Customers [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	0		4,589
Intersegment Revenues
Total operating revenues, unaffiliated customers	0		4,589
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	718	[1]	644	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	718	[1]	644	[1]
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	290	[1]	292	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	290	[1]	292	[1]
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	207	[1]	99	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	207	[1]	99	[1]
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	221	[1]	253	[1]
Intersegment Revenues
Total operating revenues, unaffiliated customers	$ 221	[1]	$ 253	[1]

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