Commitments and Contingencies - Schedule of Accrued Liabilities for Environmental Exposures (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2015 USD ($)
Commitments and Contingencies [Line Items]
Beginning balance	$ 28
Accruals	3
Payments	3
Ending balance	28
Less amounts in Other Current Liabilities	4
Amounts in Other Deferred Credits	24
Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	6
Payments	1
Ending balance	5
Less amounts in Other Current Liabilities	1
Amounts in Other Deferred Credits	4
Legacy Generation Non-Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	5
Ending balance	5
Amounts in Other Deferred Credits	5
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	19
Payments	1
Ending balance	18
Less amounts in Other Current Liabilities	2
Amounts in Other Deferred Credits	16
Potomac Electric Power Co [Member] | Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	3
Ending balance	3
Amounts in Other Deferred Credits	3
Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	3
Accruals	3
Payments	2
Ending balance	4
Less amounts in Other Current Liabilities	2
Amounts in Other Deferred Credits	2
Delmarva Power & Light Co/De [Member] | Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	2
Payments	1
Ending balance	1
Less amounts in Other Current Liabilities	1
Atlantic City Electric Co [Member] | Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	1
Ending balance	1
Amounts in Other Deferred Credits	1
Transmission and Distribution [Member]
Commitments and Contingencies [Line Items]
Beginning balance	17
Accruals	3
Payments	2
Ending balance	18
Less amounts in Other Current Liabilities	3
Amounts in Other Deferred Credits	15
Transmission and Distribution [Member] | Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	16
Payments	1
Ending balance	15
Less amounts in Other Current Liabilities	2
Amounts in Other Deferred Credits	13
Transmission and Distribution [Member] | Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	1
Accruals	3
Payments	1
Ending balance	3
Less amounts in Other Current Liabilities	1
Amounts in Other Deferred Credits	$ 2