Rates and Other Regulatory Activities - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) dekatherm / d customer mi in	Mar. 31, 2016 USD ($)
Rates and Other Regulatory Activities [Line Items]
Revenues	$ 185,160	$ 146,296
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
Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number Of Intervening Parties	14
Cost of Services	$ 60,000
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 18,900
Public Utilities, Requested Return on Equity, Percentage	13.75%
Parties Filing Protest	6
Number of Months Rates Suspended	5 months
White Oak Lateral Mainline Expansion [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost	$ 41,000
Volume The Expansion Project Is Expected to Provide | dekatherm / d	45,000
Number of pipeline miles requested | mi	7.2
Lateral diamater of pipeline to be installed | in	16
Revised Miles Of Natural Gas Pipeline | mi	5.4
System Reliability Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost	$ 37,000
Number of pipeline miles requested | mi	10.1
Lateral diamater of pipeline to be installed | in	16
Number Of Environmental Conditions | mi	19
2017 Expansion Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Estimated Capital Cost	$ 98,600
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
Subscribers [Member] | 2017 Expansion Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number of Customers intervened | customer	6
Number of customers	4