Regulatory Matters - Schedule of Current Base Rate Approval to Increase Annual Revenue (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	9 Months Ended
	Jul. 12, 2013 Potomac Electric Power Co [Member]	Sep. 30, 2014 Potomac Electric Power Co [Member] District of Columbia [Member]	Sep. 30, 2014 Potomac Electric Power Co [Member] MD [Member]	Sep. 30, 2014 Delmarva Power Light Co Electric [Member] DE [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member] NJ [Member]
Public Utilities, General Disclosures [Line Items]
Approved Revenue Requirement Increase	$ 27.9	$ 23.4	$ 8.8	$ 15.1	$ 19.0
Approved Return on Equity	9.36%	9.40%	9.62%	9.70%	9.75%
Completion Date		Mar. 26, 2014	Jul. 02, 2014	Apr. 02, 2014	Aug. 20, 2014
Rate Effective Date		Apr. 16, 2014	Jul. 04, 2014	May 01, 2014	Sep. 01, 2014