Rates and Other Regulatory Activities - Additional Information 1 (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 DTH	May 14, 2012
Investment In Affiliates [Line Items]
Additional firm transportation service capacity	17,500
Eastern Shore Mainline Expansion Project [Member]
Investment In Affiliates [Line Items]
Estimated capital cost of Mainline Expansion Project		16.3
Eastern Shore Gas Company [Member]
Investment In Affiliates [Line Items]
Estimated capital costs of project	12.1
Eastern Shore Gas Company [Member] | Eastern Shore Mainline Expansion Project [Member]
Investment In Affiliates [Line Items]
Approval for volume of additional firm service to construct, own and operate the facilities	15,040
Extension requested by protesting non-profit organization for public review and comment		60 days