Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	$ 104,567	$ 91,913	$ 92,682	$ 170,081	$ 120,380	$ 91,619	$ 100,497	$ 186,337	$ 459,244	$ 498,834	$ 444,306
Intersegment Revenues (1)
Total intersegment revenues	104,567	91,913	92,682	170,081	120,380	91,619	100,497	186,337	459,244	498,834	444,306
Operating Income
Operating Income	16,171	10,909	13,170	37,508	12,408	7,792	10,457	31,623	77,758	62,279	62,734
Gains from sales of businesses									0	7,139	0
Other income									293	101	372
Interest charges									10,006	9,482	8,234
Income Before Income taxes									68,045	60,037	54,872
Income taxes									26,905	23,945	22,085
Net Income	8,619	$ 5,119	$ 6,294	$ 21,109	10,097	$ 3,180	$ 5,134	$ 17,681	41,140	36,092	32,787
Depreciation and Amortization
Total depreciation and amortization									29,972	26,316	23,965
Capital Expenditures
Total capital expenditures									142,713	98,057	108,039
Identifiable Assets
Total identifiable assets	1,068,586				904,469				1,068,586	904,469
Regulated Energy
Operating Income
Operating Income									60,985	50,451	50,084
Depreciation and Amortization
Total depreciation and amortization									24,195	21,915	19,822
Capital Expenditures
Total capital expenditures									98,372	84,959	95,944
Identifiable Assets
Total identifiable assets	870,559				796,021				870,559	796,021
Unregulated Energy
Operating Income
Operating Income									16,355	11,723	12,353
Depreciation and Amortization
Total depreciation and amortization									5,679	3,994	3,686
Capital Expenditures
Total capital expenditures									38,347	9,648	4,829
Identifiable Assets
Total identifiable assets	172,803				84,732				172,803	84,732
Other
Operating Income
Operating Income									418	105	297
Capital Expenditures
Total capital expenditures									5,994	3,450	7,266
Identifiable Assets
Total identifiable assets	$ 25,224				$ 23,716				25,224	23,716
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization									98	407	457
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									459,244	498,834	444,306
Intersegment Revenues (1)
Total intersegment revenues									459,244	498,834	444,306
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									300,674	299,345	263,573
Intersegment Revenues (1)
Total intersegment revenues									300,674	299,345	263,573
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									158,570	184,557	161,760
Intersegment Revenues (1)
Total intersegment revenues									158,570	184,557	161,760
Operating Segments | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									0	14,932	18,973
Intersegment Revenues (1)
Total intersegment revenues									0	14,932	18,973
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									5,645	2,480	7,044
Intersegment Revenues (1)
Total intersegment revenues									5,645	2,480	7,044
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									1,228	1,097	1,064
Intersegment Revenues (1)
Total intersegment revenues									1,228	1,097	1,064
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									3,537	404	4,963
Intersegment Revenues (1)
Total intersegment revenues									3,537	404	4,963
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									880	979	1,017
Intersegment Revenues (1)
Total intersegment revenues									$ 880	$ 979	$ 1,017