Fair Value Disclosures - Gains on Level 3 Instruments Included in Income (Detail) (Other Income [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	$ 3	$ 4
Change in unrealized gains relating to assets still held at reporting date	3	4
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	4
Change in unrealized gains relating to assets still held at reporting date	$ 3	$ 4