Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 201,190	$ 140,279	$ 136,664	$ 239,356	$ 180,403	$ 126,936	$ 125,084	$ 185,160	$ 717,489	$ 617,583	$ 498,860
Intersegment Revenues (1)
Total intersegment revenues		201,190	140,279	136,664	239,356	180,403	126,936	125,084	185,160	717,489	617,583	498,860
Operating Income
Operating Income		28,930	12,036	13,248	40,406	23,628	14,632	14,061	35,099	94,620	87,420	85,983
Other income										(615)	(2,342)	(2,328)
Interest charges										16,431	12,645	10,639
Income Before Income taxes										77,574	72,433	73,016
Income taxes										20,994	14,309	28,341
Net Income		17,801	$ 5,538	$ 6,387	$ 26,855	26,101	$ 6,833	$ 6,046	$ 19,144	56,580	58,124	44,675
Depreciation and Amortization
Total depreciation and amortization										40,802	36,599	32,159
Identifiable Assets
Total identifiable assets		1,693,671				1,414,934				1,693,671	1,414,934
Regulated Energy
Operating Income
Operating Income										79,215	74,584	71,515
Depreciation and Amortization
Total depreciation and amortization										31,876	28,554	25,677
Payments to Acquire Productive Assets										235,912	159,011	139,994
Identifiable Assets
Total identifiable assets		1,345,805				1,121,673				1,345,805	1,121,673
Unregulated Energy
Operating Income
Operating Income										16,901	12,631	14,066
Depreciation and Amortization
Total depreciation and amortization										8,845	7,954	6,386
Payments to Acquire Productive Assets										38,700	26,190	23,984
Identifiable Assets
Total identifiable assets		306,045				259,041				306,045	259,041
Other
Operating Income
Operating Income										(1,496)	205	402
Depreciation and Amortization
Payments to Acquire Productive Assets										8,364	5,902	5,398
Identifiable Assets
Total identifiable assets		$ 41,821				$ 34,220				41,821	34,220
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization										81	91	96
Total for Segments [Member]
Depreciation and Amortization
Payments to Acquire Productive Assets										282,976	191,103	169,376
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										717,489	617,583	498,860
Intersegment Revenues (1)
Total intersegment revenues										717,489	617,583	498,860
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										332,749	316,971	302,402
Intersegment Revenues (1)
Total intersegment revenues										332,749	316,971	302,402
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										384,740	300,612	196,458
Intersegment Revenues (1)
Total intersegment revenues										384,740	300,612	196,458
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									49,062	34,096	11,488
Intersegment Revenues (1)
Total intersegment revenues	[1]									49,062	34,096	11,488
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									9,339	12,532	3,287
Intersegment Revenues (1)
Total intersegment revenues	[1]									9,339	12,532	3,287
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									23,983	35,877	7,321
Intersegment Revenues (1)
Total intersegment revenues	[1]									23,983	35,877	7,321
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									774	653	880
Intersegment Revenues (1)
Total intersegment revenues	[1]									$ 774	$ 653	$ 880

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