Stock-Based Compensation, Dividend Restrictions, and Calculations of Earnings Per Share of Common Stock - Dividends Received from Subsidiaries (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	$ 212	$ 136	$ 70
Potomac Electric Power Co [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	86	46	35
Delmarva Power & Light Co/De [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	100	30
Atlantic City Electric Co [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	$ 26	$ 60	$ 35