Commitments and Contingencies - Environmental Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended				1 Months Ended		9 Months Ended
	Jan. 31, 2011 gal	Mar. 31, 2013	Sep. 30, 2013 gal	Jun. 30, 2012 PHI [Member]	Sep. 30, 2013 PHI [Member]	Sep. 30, 2013 PHI [Member] Maximum [Member]	Sep. 30, 2013 PHI [Member] Maximum [Member] Brandywine Fly Ash Disposal Site [Member]	Sep. 30, 2013 PHI [Member] Minimum [Member]	Jun. 30, 2012 Potomac Electric Power Co [Member]	Jan. 31, 2011 Potomac Electric Power Co [Member] gal	Sep. 30, 2013 Potomac Electric Power Co [Member] gal	Sep. 30, 2013 Potomac Electric Power Co [Member] Brandywine Fly Ash Disposal Site [Member]	Sep. 30, 2013 Potomac Electric Power Co [Member] Maximum [Member]	Sep. 30, 2013 Potomac Electric Power Co [Member] Minimum [Member]	Sep. 30, 2013 Atlantic City Electric Co [Member]
Commitments and Contingencies [Line Items]
EPA costs to date to clean up site			$ 6												$ 6
Estimated costs remaining to remediate the site			6												6
Quantity of mineral oil spill (gallons)	4,500									4,500
Civil penalty and Supplement Environmental Projects cost				1					1
Period of capping of the right-of-way							90 days
Cost of implementation of a closure plan and cap						6		3					6	3
Quantity of fluid (Gallons)			11,250								11,250
Expense incurred on Damage					1						1
Tax payment made	74
Income tax penalties	1	0
Interest expense assessed relating to disallowed deductions	28
Non-cash charge (after-tax)		377
Maximum tax penalty percentage		20.00%
Federal income tax benefits		192
Percentage of disallowed tax benefits associated with leases		100.00%
Potential interest on potential tax liability related to disallowed tax benefits		50
Deposit of additional taxes and related interest		$ 242
Period of capping of the right-of-way												90 days