Rates and Other Regulatory Activities - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) dekatherm / d unit mi	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) dekatherm / d unit customer in mi	Jun. 30, 2016 USD ($)
Rates and Other Regulatory Activities [Line Items]
Revenues	$ 125,084	$ 102,342	$ 310,244	$ 248,638
Federal Energy Regulatory Commission Comment Period			30 days
Number Of Federal and State Bodies To Submit Comments | unit			4
Number of Private Parties To Submit Comments | unit			2
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		7.2
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
Number Of Environmental Conditions | unit	22		22
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		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 intervened | customer			6
Number of customers			7
Number of affiliates | unit			3