Commitments and Contingencies - Environmental Matters - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	9 Months Ended
	Jan. 31, 2011 gal	Mar. 31, 2013	Sep. 30, 2014
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
Civil penalty payment			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%
Percentage of disallowed tax benefits associated with leases		100.00%
Federal income tax benefits		192,000,000
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
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
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Quantity of mineral oil spill	4,500
Cost of litigation			875,000
Civil penalty payment			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