Rates and Other Regulatory Activities - Additional Information (Detail) $ in Thousands		1 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended
	Feb. 02, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) mi	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) mi	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 USD ($) mi	Dec. 31, 2017 USD ($) dekatherm / d unit mi in	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Rates and Other Regulatory Activities [Line Items]
Restricted Payment			$ 178,000									$ 178,000
Regulatory Assets			86,505				$ 84,497					86,505	$ 84,497
Revenues			180,403	$ 126,936	$ 125,084	$ 185,160	$ 141,874	$ 108,348	$ 102,342	$ 146,296		617,583	$ 498,860	$ 459,244
Electric distribution
Rates and Other Regulatory Activities [Line Items]
Amount of Regulatory Costs Not yet Approved			$ 15,200									15,200
DELAWARE
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount												$ 2,250
Florida Public Utilities Company [Member] | New Smyrna Beach [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi												14.0
Florida Public Utilities Company [Member] | Northwest Florida Expansion [Member]
Rates and Other Regulatory Activities [Line Items]
Lateral Diameter Of Pipeline To Be Installed | in												8
Diameter of pipe to be installed in the future | mi			12.0									12.0
Florida
Rates and Other Regulatory Activities [Line Items]
Estimated capital costs of project	$ 59,800
Number Of Years In Project	5 years
Florida | New Smyrna Beach [Member]
Rates and Other Regulatory Activities [Line Items]
number of customers												2
Florida | Belvedere Project [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi												2.0
Eastern Shore Gas Company
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount												$ 9,800
Cost of Services		$ 60,000
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 18,900
Public Utilities, Requested Return on Equity, Percentage		13.75%
Number of Months Rates Suspended		5 months
Increase in Revenue Recognized Due to Motion Rate in Effect			$ 3,700									3,700
Eastern Shore Gas Company | White Oak Lateral Mainline Expansion [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated capital costs of project												$ 42,000
Lateral Diameter Of Pipeline To Be Installed | in												16
Volume The Expansion Project Is Expected to Provide | dekatherm / d												45,000
Miles Of Natural Gas Pipeline | mi					5.4						5.4
Eastern Shore Gas Company | System Reliability Project [Member]
Rates and Other Regulatory Activities [Line Items]
Number of Pipeline Miles | mi			10.1									10.1
Estimated capital costs of project											$ 38,000
Lateral Diameter Of Pipeline To Be Installed | in												16
Eastern Shore Gas Company | 2017 Expansion Project [Member]
Rates and Other Regulatory Activities [Line Items]
firm natural gas transportation deliverability | dekatherm / d												61,162
Additional Firm Natural Gas Transportation Deliverability | dekatherm / d												52,500
Number of Pipeline Miles | mi			23									23
Estimated capital costs of project												$ 117,000
Miles Of Natural Gas Pipeline | mi			17									17
Pressure Control Stations											2
Florida Public Utilities Company [Member]
Rates and Other Regulatory Activities [Line Items]
Restricted Payment			$ 2,500									$ 2,500
Business Acquisition Premium Paid			$ 34,200									$ 34,200
Subscribers [Member] | Eastern Shore Gas Company | 2017 Expansion Project [Member]
Rates and Other Regulatory Activities [Line Items]
number of customers												7
Number of affiliates | unit												3
12 inches [Member] | Florida Public Utilities Company [Member] | Northwest Florida Expansion [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi												33.0
8 inches [Member] | Florida Public Utilities Company [Member] | Northwest Florida Expansion [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi												8.0