Income Taxes - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Feb. 26, 2015	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2015
Schedule of Income Tax [Line Items]
Tax benefit related to certain energy efficiency tax deductions		$ 2		$ 6
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]
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%