Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Aug. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 02, 2013	Aug. 30, 2010	Oct. 31, 2009
Goodwill [Line Items]
Goodwill		$ 22,104	$ 15,070
Amortization of intangible assets		537	380	$ 367
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		790
Amortization of intangible assets, 2021		725
Amortization of intangible assets, 2022		471
Regulated Energy
Goodwill [Line Items]
Goodwill		3,353	3,353
Unregulated Energy
Goodwill [Line Items]
Goodwill		18,751	$ 11,717
Goodwill, Acquired During Period	$ 1,900
FPU | Regulated Energy
Goodwill [Line Items]
Goodwill							$ 2,500
Indiantown Gas Company | Regulated Energy
Goodwill [Line Items]
Goodwill						$ 170
Fort Meade
Goodwill [Line Items]
Goodwill					$ 714
Gatherco
Goodwill [Line Items]
Goodwill				$ 10,119
Gatherco | Unregulated Energy
Goodwill [Line Items]
Goodwill		$ 10,100
ARM [Member] | Unregulated Energy
Goodwill [Line Items]
Goodwill, Acquired During Period	$ 6,800
Anderson Gas
Goodwill [Line Items]
Amortized period of acquired intangible assets		6 years
Virginia LP Gas, Inc.
Goodwill [Line Items]
Amortized period of acquired intangible assets		7 years
Virginia LP Gas, Inc. | Maximum
Goodwill [Line Items]
Amortized period of acquired intangible assets		40 years
Customer list | Minimum
Goodwill [Line Items]
Amortized period of acquired intangible assets		7 years
Customer list | Maximum
Goodwill [Line Items]
Amortized period of acquired intangible assets		12 years