Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Sep. 30, 2014	Mar. 31, 2013	Dec. 31, 2013
Schedule of Income Tax [Line Items]
Tax benefit related to certain energy efficiency tax deductions	$ 4
Change in estimates and interest related to uncertain tax positions		51
Establishment of valuation allowances on deferred tax assets		101
PCI future deferred tax asset		101	101
After-tax effect of revised lease rerun		377
Potomac Electric Power Co [Member]
Schedule of Income Tax [Line Items]
After-tax non-cash charge		54
Additional interest income		5
After-tax effect of revised lease rerun		377
Delmarva Power & Light Co/De [Member]
Schedule of Income Tax [Line Items]
After-tax non-cash charge		54
Additional interest income		1
After-tax effect of revised lease rerun		377
Atlantic City Electric Co [Member]
Schedule of Income Tax [Line Items]
After-tax non-cash charge		54
Additional interest income		6
After-tax effect of revised lease rerun		$ 377