Rates and Other Regulatory Activities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($) dekatherm / d unit mi	Dec. 31, 2017 USD ($)
Electric Distribution [Member]
Rates and Other Regulatory Activities [Line Items]
Amount of Regulatory Costs Not yet Approved	$ 15,200,000
Electric Limited Proceedings [Member]
Rates and Other Regulatory Activities [Line Items]
Asset Recovery Damaged Property Costs, Noncurrent	1,500,000
Deferred Storm and Property Reserve Deficiency, Noncurrent	779,000
Public Utilities, Requested Rate Increase (Decrease), Amount	1.82
Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Cost of Services	60,000,000
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 18,900,000
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		$ 3,700,000
2017 Expansion Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost	$ 117,000,000
Number of pipeline miles requested | mi	23
Revised Miles Of Natural Gas Pipeline | mi	17
Pressure Control Stations	2
Firm natural gas transportation deliverability | dekatherm / d	61,162
Additional Firm Natural Gas Transportation Deliverability | dekatherm / d	52,500
Del-Mar Pathway Project [Domain] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Firm natural gas transportation deliverability | dekatherm / d	14,300
Del-Mar Pathway Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number of pipeline miles requested | mi	13
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
Pre TCJA approved revenue requirements [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 9,800,000
Post TCJA approved revenue requirements [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 6,600,000