Commitments and Contingencies - Schedule of Accrued Liabilities for Environmental Exposures (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Commitments and Contingencies [Line Items]
Beginning balance	$ 30
Payments	2
Ending balance	28
Less amounts in Other Current Liabilities	4
Amounts in Other Deferred Credits	24
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	21
Payments	2
Ending balance	19
Less amounts in Other Current Liabilities	2
Amounts in Other Deferred Credits	17
Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	3
Ending balance	3
Less amounts in Other Current Liabilities	2
Amounts in Other Deferred Credits	1
Transmission and Distribution [Member]
Commitments and Contingencies [Line Items]
Beginning balance	19
Payments	2
Ending balance	17
Less amounts in Other Current Liabilities	3
Amounts in Other Deferred Credits	14
Transmission and Distribution [Member] | Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	18
Payments	2
Ending balance	16
Less amounts in Other Current Liabilities	2
Amounts in Other Deferred Credits	14
Transmission and Distribution [Member] | Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	1
Ending balance	1
Less amounts in Other Current Liabilities	1
Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	6
Ending balance	6
Less amounts in Other Current Liabilities	1
Amounts in Other Deferred Credits	5
Legacy Generation Regulated [Member] | Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	3
Ending balance	3
Amounts in Other Deferred Credits	3
Legacy Generation Regulated [Member] | Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	2
Ending balance	2
Less amounts in Other Current Liabilities	1
Amounts in Other Deferred Credits	1
Legacy Generation Regulated [Member] | Atlantic City Electric Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	1
Ending balance	1
Amounts in Other Deferred Credits	1
Legacy Generation Non-Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	5
Ending balance	5
Amounts in Other Deferred Credits	$ 5