Commitments and Contingencies - Tax Legislation, Guarantees, Indemnifications, and Performance Contracts - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Commitments [Line Items]
Contractual obligations due in 2016	$ 268
Contractual obligations due in 2017 to 2018	469
Contractual obligations due in 2019 to 2020	472
Contractual obligations due, thereafter	993
Rental expense for operating leases	60	$ 59	$ 54
Future minimum operating lease payments, 2016	52
Future minimum operating lease payments, 2017	48
Future minimum operating lease payments, 2018	47
Future minimum operating lease payments, 2019	34
Future minimum operating lease payments, 2020	37
Future minimum operating lease payments, thereafter	$ 304
Pepco Energy Services [Member] | Energy Savings or Combined Heat and Power Performance [Member]
Other Commitments [Line Items]
Contract life, maximum remaining term	20 years
Value of guarantees under construction projects	$ 15
Accrued liability on contracts	1
Pepco Energy Services [Member] | Completed Performance Contracts Associated With Savings Guarantees [Member]
Other Commitments [Line Items]
Value of guarantees completed projects	$ 221
Contract life, maximum remaining term	23 years
Pepco Energy Services [Member] | Uncompleted Performance Contracts Associated With Savings Guarantees [Member]
Other Commitments [Line Items]
Value of guarantees on projects under construction	$ 55
Maximum term of project under construction	19 years
Potomac Electric Power Co [Member]
Other Commitments [Line Items]
Rental expense for operating leases	$ 7	8	7
Future minimum operating lease payments, 2016	7
Future minimum operating lease payments, 2017	6
Future minimum operating lease payments, 2018	5
Future minimum operating lease payments, 2019	4
Future minimum operating lease payments, 2020	3
Future minimum operating lease payments, thereafter	7
Delmarva Power & Light Co/De [Member]
Other Commitments [Line Items]
Contractual obligations due in 2016	64
Contractual obligations due in 2017 to 2018	127
Contractual obligations due in 2019 to 2020	128
Contractual obligations due, thereafter	296
Rental expense for operating leases	14	14	13
Future minimum operating lease payments, 2016	13
Future minimum operating lease payments, 2017	12
Future minimum operating lease payments, 2018	16
Future minimum operating lease payments, 2019	6
Future minimum operating lease payments, 2020	9
Future minimum operating lease payments, thereafter	$ 118
Ownership interest	11.90%
Present value of future minimum lease payments	$ 75
Atlantic City Electric Co [Member]
Other Commitments [Line Items]
Contractual obligations due in 2016	203
Contractual obligations due in 2017 to 2018	341
Contractual obligations due in 2019 to 2020	344
Contractual obligations due, thereafter	697
Rental expense for operating leases	13	$ 12	$ 12
Future minimum operating lease payments, 2016	8
Future minimum operating lease payments, 2017	8
Future minimum operating lease payments, 2018	7
Future minimum operating lease payments, 2019	6
Future minimum operating lease payments, 2020	6
Future minimum operating lease payments, thereafter	$ 30