Rates and Other Regulatory Activities - Additional Information (Detail) $ in Thousands				1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 19, 2016 USD ($)	Dec. 21, 2015 USD ($)	Oct. 13, 2015 dekatherm / d	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($) mi	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) dekatherm / d in mi	Jun. 30, 2015 USD ($)	Jul. 21, 2016 mi	May 12, 2016 mi
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Interim Rate Increase (Decrease), Amount | $	$ 2,500
Revenues | $					$ 102,342	$ 92,682	$ 248,638	$ 262,763
Public Utilities Interim Requested Rate Increase (Decrease) Percentage	5.00%
DELAWARE
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount | $		$ 4,700
Public Utilities, Requested Rate Increase (Decrease), Percentage		10.00%
Revenues | $					555		878
Revenues Net Of Tax | $					$ 332		$ 526
MARYLAND
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount | $				$ 950
Public Utilities, Requested Rate Increase (Decrease), Percentage				5.00%
Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Volume The Expansion Project Is Expected to Provide | dekatherm / d			53,000
Total Capacity After Pipeline Improvements | dekatherm / d			160,000
White Oak Lateral Mainline Expansion [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Volume The Expansion Project Is Expected to Provide | dekatherm / d							45,000
Number of pipeline miles | mi					7.2		7.2
Lateral diamater of pipeline to be installed | in							16
Horsepower Of Additional Compression							3,550
Miles Of Natural Gas Pipeline | mi					5.4		5.4
System Reliability Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number of pipeline miles | 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]
Volume The Expansion Project Is Expected to Provide | dekatherm / d							86,437
Number of pipeline miles | mi										33
Horsepower Of Additional Compression							3,550
Miles Of Natural Gas Pipeline | mi										17
Subsequent Event [Member] | System Reliability Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number Of Environmental Conditions | mi									19