Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Significant Accounting Policies [Line Items]
Estimated fair value of an asset	Greater than 50%
Taxes included in gross revenues	$ 83	$ 83
Potomac Electric Power Co [Member]
Significant Accounting Policies [Line Items]
Taxes included in gross revenues	78	78
Delmarva Power & Light Co/De [Member]
Significant Accounting Policies [Line Items]
Estimated fair value of an asset	Greater than 50%
Taxes included in gross revenues	5	4
Atlantic City Electric Co [Member]
Significant Accounting Policies [Line Items]
Taxes included in gross revenues	$ 0	$ 1