Fair Value Disclosures - Gains on Level 3 Instruments Included in Income (Detail) - Other Income [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	$ 3	$ 2
Change in unrealized gains relating to assets still held at reporting date	2	2
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	3	2
Change in unrealized gains relating to assets still held at reporting date	$ 2	$ 2