Stock-Based Compensation, Dividend Restrictions, and Calculations of Earnings Per Share of Common Stock - Dividends Received from Subsidiaries (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	$ 250	$ 212	$ 136
Potomac Electric Power Co [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	146	86	46
Delmarva Power & Light Co/De [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	92	100	30
Atlantic City Electric Co [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Dividends received from subsidiaries	$ 12	$ 26	$ 60