Commitments and Contingencies - Schedule of Accrued Liabilities for Environmental Exposures (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Commitments and Contingencies [Line Items]
Beginning balance	$ 28
Accruals	5
Payments	4
Ending balance	29
Less amounts in Other Current Liabilities		$ 5
Amounts in Other Deferred Credits		24
Total accrued liabilities for environmental contingencies	29	29
Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	6
Accruals	2
Payments	1
Ending balance	7
Less amounts in Other Current Liabilities		1
Amounts in Other Deferred Credits		6
Total accrued liabilities for environmental contingencies	7	7
Legacy Generation Non-Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	5
Ending balance	5
Amounts in Other Deferred Credits		5
Total accrued liabilities for environmental contingencies	5	5
Potomac Electric Power Co [Member]
Commitments and Contingencies [Line Items]
Beginning balance	19
Accruals	2
Payments	1
Ending balance	20
Less amounts in Other Current Liabilities		3
Amounts in Other Deferred Credits		17
Total accrued liabilities for environmental contingencies	20	20
Potomac Electric Power Co [Member] | Legacy Generation Regulated [Member]
Commitments and Contingencies [Line Items]
Beginning balance	3
Accruals	2
Ending balance	5
Amounts in Other Deferred Credits		5
Total accrued liabilities for environmental contingencies	5	5
Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	3
Accruals	3
Payments	3
Ending balance	3
Less amounts in Other Current Liabilities		2
Amounts in Other Deferred Credits		1
Total accrued liabilities for environmental contingencies	3	3
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
Total accrued liabilities for environmental contingencies	1	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
Total accrued liabilities for environmental contingencies	1	1
Transmission and Distribution [Member]
Commitments and Contingencies [Line Items]
Beginning balance	17
Accruals	3
Payments	3
Ending balance	17
Less amounts in Other Current Liabilities		4
Amounts in Other Deferred Credits		13
Total accrued liabilities for environmental contingencies	17	17
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		3
Amounts in Other Deferred Credits		12
Total accrued liabilities for environmental contingencies	15	15
Transmission and Distribution [Member] | Delmarva Power & Light Co/De [Member]
Commitments and Contingencies [Line Items]
Beginning balance	1
Accruals	3
Payments	2
Ending balance	2
Less amounts in Other Current Liabilities		1
Amounts in Other Deferred Credits		1
Total accrued liabilities for environmental contingencies	$ 2	$ 2