Commitments and Contingencies - Environmental Matters - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended
	Jan. 31, 2011 gal	Mar. 31, 2014	Mar. 31, 2013
Commitments and Contingencies [Line Items]
EPA costs to date to clean up site		$ 6,000,000
Estimated costs remaining to remediate the site		6,000,000
Quantity of mineral oil spill	4,500
Cost of litigation		875,000
Litigation penalty obligation		250,000
Donation for training fund		25,000
Fund provided for installation and operation of the trash collection SEP		600,000
Tax payment made	74,000,000		74,000,000
Income tax penalties	1,000,000		0
Interest expense assessed relating to disallowed deductions	28,000,000
Non-cash charge (after-tax)			377,000,000
Maximum tax penalty percentage			20.00%
Federal income tax benefits			192,000,000
Percentage of disallowed tax benefits associated with leases			100.00%
Potential interest on potential tax liability related to disallowed tax benefits			50,000,000
Deposit of additional taxes and related interest			242,000,000
PHI [Member] | Minimum [Member]
Commitments and Contingencies [Line Items]
Cost of implementation of a closure plan and cap		3,000,000
PHI [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Cost of implementation of a closure plan and cap		6,000,000
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Quantity of mineral oil spill	4,500
Cost of litigation		875,000
Litigation penalty obligation		250,000
Donation for training fund		25,000
Fund provided for installation and operation of the trash collection SEP		600,000
Non-cash charge (after-tax)			377,000,000
Potomac Electric Power Co [Member] | Minimum [Member]
Commitments and Contingencies [Line Items]
Cost of implementation of a closure plan and cap		3,000,000
Potomac Electric Power Co [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Cost of implementation of a closure plan and cap		6,000,000
Atlantic City Electric Co [Member]
Commitments and Contingencies [Line Items]
EPA costs to date to clean up site		6,000,000
Estimated costs remaining to remediate the site		6,000,000
Non-cash charge (after-tax)			$ 377,000,000