Commitments and Contingencies - Environmental Matters - Additional Information (Detail)	1 Months Ended			3 Months Ended	6 Months Ended
	Mar. 31, 2014 USD ($)	Jan. 31, 2011 USD ($) gal	Nov. 30, 2008 USD ($)	Mar. 31, 2013 USD ($)	Jun. 30, 2015 USD ($)
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 | gal		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
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Quantity of mineral oil spill | gal		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
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