Commitments and Contingencies - Environmental Matters - Additional Information (Detail)	1 Months Ended					3 Months Ended	12 Months Ended
	Aug. 31, 2015 gal	Mar. 31, 2014 USD ($)	Jul. 31, 2013 USD ($)	Jan. 31, 2011 USD ($) gal	Nov. 30, 2008 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2015 USD ($)
Other Commitments [Line Items]
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
U.S. Environmental Protection Agency [Member]
Other Commitments [Line Items]
Costs to date to clean up site					6,000,000
Minimum [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							$ 9,000,000
Maximum [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							13,000,000
PHI [Member]
Other Commitments [Line Items]
Mineral oil released | gal	6,100
Released mineral oil recovered through remediation efforts, percentage	80.00%
PHI [Member] | Minimum [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							3,000,000
Proposed compensation for damages			$ 106,000
PHI [Member] | Maximum [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							6,000,000
Proposed compensation for damages			161,000
Potomac Electric Power Co [Member]
Other Commitments [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
Mineral oil released | gal	6,100
Released mineral oil recovered through remediation efforts, percentage	80.00%
Non-cash charge (after-tax)						377,000,000
Potomac Electric Power Co [Member] | Minimum [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							3,000,000
Proposed compensation for damages			106,000
Potomac Electric Power Co [Member] | Minimum [Member] | Environmental Protection Agency [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							8,000,000
Potomac Electric Power Co [Member] | Maximum [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							6,000,000
Proposed compensation for damages			$ 161,000
Potomac Electric Power Co [Member] | Maximum [Member] | Environmental Protection Agency [Member]
Other Commitments [Line Items]
Cost of implementation of a closure plan and cap							$ 12,000,000
Atlantic City Electric Co [Member]
Other Commitments [Line Items]
Estimated costs remaining to remediate the site					6,000,000
Non-cash charge (after-tax)						$ 377,000,000
Atlantic City Electric Co [Member] | U.S. Environmental Protection Agency [Member]
Other Commitments [Line Items]
Costs to date to clean up site					$ 6,000,000