Quarterly Financial Information - Schedule of Quarterly Financial Information (Parenthetical) (Detail) (USD $)	12 Months Ended	3 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2014
Quarterly Financial Information [Line Items]
Establishment of valuation allowances related to deferred tax assets	$ 101,000,000
Pepco Energy Services [Member]
Quarterly Financial Information [Line Items]
Reversed unbilled revenue, pre-tax		3,000,000
Reversed unbilled revenue, after-tax		2,000,000
Impairment losses, pre-tax		28,000,000	53,000,000	4,000,000
Charge to correct prior period error before tax		3,000,000
Impairment losses, after-tax		16,000,000	32,000,000	3,000,000
Change in estimates and interest related to uncertain and effectively settled tax positions				56,000,000
Establishment of valuation allowances related to deferred tax assets				101,000,000
Charge to correct prior period error after-tax		2,000,000		4,000,000
Potomac Electric Power Co [Member]
Quarterly Financial Information [Line Items]
Establishment of valuation allowances related to deferred tax assets	0					0
Charge to correct prior period error after-tax				4,000,000
Tax benefits related to uncertain tax position IRS					5,000,000
Atlantic City Electric Co [Member]
Quarterly Financial Information [Line Items]
Reversed unbilled revenue, pre-tax		3,000,000
Reversed unbilled revenue, after-tax		2,000,000
Charge to correct prior period error before tax		3,000,000
Establishment of valuation allowances related to deferred tax assets	0					0
Charge to correct prior period error after-tax		$ 2,000,000