Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 141,874	$ 108,348	$ 102,342	$ 146,296	$ 104,567	$ 91,913	$ 92,682	$ 170,081	$ 498,860	$ 459,244	$ 498,834
Intersegment Revenues (1)
Total intersegment revenues		141,874	108,348	102,342	146,296	104,567	91,913	92,682	170,081	498,860	459,244	498,834
Operating Income
Operating Income		21,819	10,156	15,742	36,380	16,171	10,909	13,170	37,508	84,096	77,758	62,279
Gains from sales of businesses										0	0	7,139
Other income										(441)	293	101
Interest charges										10,639	10,006	9,482
Income Before Income taxes										73,016	68,045	60,037
Income taxes										28,341	26,905	23,945
Net Income		11,863	$ 4,416	$ 8,029	$ 20,367	8,619	$ 5,119	$ 6,294	$ 21,109	44,675	41,140	36,092
Depreciation and Amortization
Total depreciation and amortization										32,159	29,972	26,316
Capital Expenditures
Total capital expenditures										169,376	142,713	98,057
Identifiable Assets
Total identifiable assets		1,229,219				1,067,421				1,229,219	1,067,421
Regulated Energy
Operating Income
Operating Income										69,851	60,985	50,451
Depreciation and Amortization
Total depreciation and amortization										25,677	24,195	21,915
Capital Expenditures
Total capital expenditures										139,994	98,372	84,959
Identifiable Assets
Total identifiable assets		986,752				872,065				986,752	872,065
Unregulated Energy
Operating Income
Operating Income										13,844	16,355	11,723
Depreciation and Amortization
Total depreciation and amortization										6,386	5,679	3,994
Capital Expenditures
Total capital expenditures										23,984	38,347	9,648
Identifiable Assets
Total identifiable assets		226,368				171,840				226,368	171,840
Other
Operating Income
Operating Income										401	418	105
Capital Expenditures
Total capital expenditures										5,398	5,994	3,450
Identifiable Assets
Total identifiable assets		$ 16,099				$ 23,516				16,099	23,516
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization										96	98	407
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										498,860	459,244	498,834
Intersegment Revenues (1)
Total intersegment revenues										498,860	459,244	498,834
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										302,402	300,674	299,345
Intersegment Revenues (1)
Total intersegment revenues										302,402	300,674	299,345
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										196,458	158,570	184,557
Intersegment Revenues (1)
Total intersegment revenues										196,458	158,570	184,557
Operating Segments | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										0	0	14,932
Intersegment Revenues (1)
Total intersegment revenues										0	0	14,932
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									11,488	5,645	2,480
Intersegment Revenues (1)
Total intersegment revenues	[1]									11,488	5,645	2,480
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									3,287	1,228	1,097
Intersegment Revenues (1)
Total intersegment revenues	[1]									3,287	1,228	1,097
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									7,321	3,537	404
Intersegment Revenues (1)
Total intersegment revenues	[1]									7,321	3,537	404
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									880	880	979
Intersegment Revenues (1)
Total intersegment revenues	[1]									$ 880	$ 880	$ 979

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