Fair Value Disclosures - Gains on Level 3 Instruments Included in Income (Detail) - Other Income [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains included in income for the period	$ 20	$ 3
Change in unrealized gains relating to assets still held at reporting date	18	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	5	3
Change in unrealized gains relating to assets still held at reporting date	$ 3	$ 3