Rates and Other Regulatory Activities - Additional Information (Detail) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) dekatherm / d in mi	Sep. 30, 2017 USD ($) dekatherm / d unit customer in mi	Dec. 31, 2016 USD ($)
DELAWARE
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 2,250
Electric Distribution [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost		$ 59,800
Number of Years in Project		5 years
Amount of Regulatory Costs Not yet Approved	$ 15,200	$ 15,200
Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
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	$ 1,000	$ 1,000
Northwest Florida Expansion [Member] | Florida Public Utilities Company [Member]
Rates and Other Regulatory Activities [Line Items]
Lateral diamater of pipeline to be installed | in		8
Diameter of pipe to be installed in the future | in	12	12
New Smyrna Beach [Member] | Florida Public Utilities Company [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi		14
White Oak Lateral Mainline Expansion [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost		$ 42,000
Volume The Expansion Project Is Expected to Provide | dekatherm / d		45,000
Lateral diamater of pipeline to be installed | in		16
Revised Miles Of Natural Gas Pipeline | mi	5.4	5.4
System Reliability Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost		$ 38,000
Number of pipeline miles requested | mi	10.1	10.1
Lateral diamater of pipeline to be installed | in		16
2017 Expansion Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost		$ 115,000
Number of pipeline miles requested | mi	23	23
Revised Miles Of Natural Gas Pipeline | mi	17	17
Pressure Control Stations		2
Firm natural gas transportation deliverability | dekatherm / d	61,162
Additional Firm Natural Gas Transportation Deliverability | dekatherm / d		52,500
Subscribers [Member] | 2017 Expansion Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number of customers		7
Number of affiliates | unit		3
Number of Customers intervened | customer		6
12 inches [Member] | Northwest Florida Expansion [Member] | Florida Public Utilities Company [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi		33
8 inches [Member] | Northwest Florida Expansion [Member] | Florida Public Utilities Company [Member]
Rates and Other Regulatory Activities [Line Items]
Length of Natural Gas Pipeline | mi		8