Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 180,403	$ 126,936	$ 125,084	$ 185,160	$ 141,874	$ 108,348	$ 102,342	$ 146,296	$ 617,583	$ 498,860	$ 459,244
Intersegment Revenues (1)
Total intersegment revenues		180,403	126,936	125,084	185,160	141,874	108,348	102,342	146,296	617,583	498,860	459,244
Operating Income
Operating Income		23,263	14,239	13,666	34,676	21,819	10,156	15,742	36,380	85,843	84,096	77,758
Other income										(765)	(441)	293
Interest charges										12,645	10,639	10,006
Income Before Income taxes										72,433	73,016	68,045
Income taxes										14,309	28,341	26,905
Net Income		26,101	$ 6,833	$ 6,046	$ 19,144	11,863	$ 4,416	$ 8,029	$ 20,367	58,124	44,675	41,140
Depreciation and Amortization
Total depreciation and amortization										36,599	32,159	29,972
Capital Expenditures
Total capital expenditures										191,103	169,376	195,261
Identifiable Assets
Total identifiable assets		1,417,434				1,229,219				1,417,434	1,229,219
Regulated Energy
Operating Income
Operating Income										73,160	69,851	60,985
Depreciation and Amortization
Total depreciation and amortization										28,554	25,677	24,195
Capital Expenditures
Total capital expenditures										159,011	139,994	98,372
Identifiable Assets
Total identifiable assets		1,121,673				986,752				1,121,673	986,752
Unregulated Energy
Operating Income
Operating Income										12,477	13,844	16,355
Depreciation and Amortization
Total depreciation and amortization										7,954	6,386	5,679
Capital Expenditures
Total capital expenditures										26,190	23,984	90,895
Identifiable Assets
Total identifiable assets		261,541				226,368				261,541	226,368
Other
Operating Income
Operating Income										206	401	418
Capital Expenditures
Total capital expenditures										5,902	5,398	5,994
Identifiable Assets
Total identifiable assets		$ 34,220				$ 16,099				34,220	16,099
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization										91	96	98
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										617,583	498,860	459,244
Intersegment Revenues (1)
Total intersegment revenues										617,583	498,860	459,244
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										316,971	302,402	300,674
Intersegment Revenues (1)
Total intersegment revenues										316,971	302,402	300,674
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues										300,612	196,458	158,570
Intersegment Revenues (1)
Total intersegment revenues										300,612	196,458	158,570
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									34,096	11,488	5,645
Intersegment Revenues (1)
Total intersegment revenues	[1]									34,096	11,488	5,645
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									9,339	3,287	1,228
Intersegment Revenues (1)
Total intersegment revenues	[1]									9,339	3,287	1,228
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									23,983	7,321	3,537
Intersegment Revenues (1)
Total intersegment revenues	[1]									23,983	7,321	3,537
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]									774	880	880
Intersegment Revenues (1)
Total intersegment revenues	[1]									$ 774	$ 880	$ 880

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