Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 26, 2015
Schedule of Income Tax [Line Items]
Consolidated effective tax rates	36.10%	38.00%
Tax benefit related to certain energy efficiency tax deductions	$ 4
Reduction in deferred tax liabilities	23
District of Columbia [Member]
Schedule of Income Tax [Line Items]
Corporate tax rate			9.975%
District of Columbia [Member] | 2019 [Member]
Schedule of Income Tax [Line Items]
Corporate tax rate			8.25%
District of Columbia [Member] | 2015 [Member]
Schedule of Income Tax [Line Items]
Corporate tax rate			9.40%
Potomac Electric Power Co [Member]
Schedule of Income Tax [Line Items]
Consolidated effective tax rates	31.60%	33.30%
Reduction in deferred tax liabilities	$ 23
Potomac Electric Power Co [Member] | District of Columbia [Member]
Schedule of Income Tax [Line Items]
Corporate tax rate			9.975%
Potomac Electric Power Co [Member] | District of Columbia [Member] | 2019 [Member]
Schedule of Income Tax [Line Items]
Corporate tax rate			8.25%
Potomac Electric Power Co [Member] | District of Columbia [Member] | 2015 [Member]
Schedule of Income Tax [Line Items]
Corporate tax rate			9.40%
Delmarva Power & Light Co/De [Member]
Schedule of Income Tax [Line Items]
Consolidated effective tax rates	39.60%	40.30%
Atlantic City Electric Co [Member]
Schedule of Income Tax [Line Items]
Consolidated effective tax rates	33.30%	37.50%