Rates and Other Regulatory Activities - Additional Information (Detail) $ in Thousands				1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 19, 2016 USD ($)	Dec. 21, 2015 USD ($)	Oct. 13, 2015 dekatherm / d	Dec. 31, 2015 USD ($)	Sep. 30, 2016 USD ($) mi	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) dekatherm / d mi in	Sep. 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 | $					$ 108,348	$ 91,913	$ 356,986	$ 354,676
Final Miles of Pipeline							23
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 | $					469		$ 1,350
Revenues Net Of Tax | $					$ 280		$ 817
Public Utilities Interim Requested Rate Increase (Decrease) Percentage	5.00%
MARYLAND
Rates and Other Regulatory Activities [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount | $				$ 950
Public Utilities Interim 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					7.2		7.2
Lateral diamater of pipeline to be installed | in							16
Horsepower Of Additional Compression							3,550
Miles Of Natural Gas Pipeline					5.4		5.4
System Reliability Project [Member] | Eastern Shore [Member]
Rates and Other Regulatory Activities [Line Items]
Number of pipeline miles					10.1		10.1
Lateral diamater of pipeline to be installed | in							16
Number Of Environmental Conditions					1		1		19
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										33
Horsepower Of Additional Compression							3,550
Miles Of Natural Gas Pipeline										17
Pressure Maintenance Operations							2
number of customers							7
firm natural gas transportation deliverability | dekatherm / d							61,162