Fair Value Disclosures - Gains on Level 3 Instruments Included in Income (Detail) - Other Income [Member] - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	$ 19	$ 3
Change in unrealized gains relating to assets still held at reporting date	17	3
Potomac Electric Power Co [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	4	3
Change in unrealized gains relating to assets still held at reporting date	$ 2	$ 3