Fair Value Disclosures - Gains or (Losses) on Level 3 Instruments Included in Income (Detail) (Other Income [Member], USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	$ 2	$ 3
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	2	3
Change in unrealized gains relating to assets still held at reporting date	$ 2	$ 2