Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 146,296	$ 170,081
Intersegment Revenues
Total operating revenues, unaffiliated customers		146,296	170,081
Operating Income
Total operating income		36,380	37,508
Other income, net of other expenses		(34)	133
Interest		2,650	2,448
Income Before Income Taxes		33,696	35,193
Income taxes		13,329	14,084
Net Income		20,367	21,109	$ 41,140
Identifiable Assets
Total identifiable assets		1,081,984		1,067,421
Regulated Energy [Member]
Operating Income
Total operating income		24,319	22,182
Identifiable Assets
Total identifiable assets		879,878		872,065
Unregulated Energy [Member]
Operating Income
Total operating income		11,936	15,229
Identifiable Assets
Total identifiable assets		178,723		171,840
Other [Member]
Identifiable Assets
Total identifiable assets		23,383		$ 23,516
Other and eliminations [Member]
Operating Income
Total operating income		125	97
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		146,296	170,081
Intersegment Revenues
Total operating revenues, unaffiliated customers		146,296	170,081
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		88,894	109,292
Intersegment Revenues
Total operating revenues, unaffiliated customers		88,894	109,292
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		57,402	60,789
Intersegment Revenues
Total operating revenues, unaffiliated customers		57,402	60,789
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	661	718
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	661	718
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	322	290
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	322	290
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	113	207
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	113	207
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	226	221
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 226	$ 221

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