Fair Value Disclosures - Gains on Level 3 Instruments Included in Income (Detail) (Other Income [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	$ 1	$ 1
Change in unrealized gains relating to assets still held at reporting date	1	1
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	1	1
Change in unrealized gains relating to assets still held at reporting date	$ 1	$ 1