Income Taxes - Reconciliation of Consolidated Income Tax Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 49	$ 40	$ 106	$ 116
State income taxes, net of Federal effect	8	6	18	21
Asset removal costs	(3)	(4)	(11)	(9)
Energy efficiency-related tax deductions	(2)	(4)	(6)	(4)
Merger-related costs			4	7
Other, net	(3)	(4)	(5)	(6)
Consolidated income tax expense	$ 49	$ 34	$ 106	$ 125
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	5.70%	5.30%	5.90%	6.30%
Asset removal costs, percentage	(2.10%)	(3.50%)	(3.60%)	(2.70%)
Energy efficiency-related tax deductions, percentage	(1.40%)	(3.50%)	(2.00%)	(1.20%)
Merger related costs, percentage			1.30%	2.10%
Other, net, percentage	(2.20%)	(3.20%)	(1.60%)	(1.90%)
Consolidated income tax expense related to continuing operations, percentage	35.00%	30.10%	35.00%	37.60%
Potomac Electric Power Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 32	$ 37	$ 67	$ 79
State income taxes, net of Federal effect	5	6	10	13
Asset removal costs	(3)	(4)	(11)	(9)
Other, net	(2)	(1)	(4)	(1)
Consolidated income tax expense	$ 32	$ 38	$ 62	$ 82
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	5.40%	5.70%	5.30%	5.70%
Asset removal costs, percentage	(3.30%)	(3.80%)	(5.80%)	(4.00%)
Other, net, percentage	(2.30%)	(0.70%)	(1.90%)	(0.60%)
Consolidated income tax expense related to continuing operations, percentage	34.80%	36.20%	32.60%	36.10%
Delmarva Power & Light Co/De [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 8	$ 13	$ 32	$ 46
State income taxes, net of Federal effect	1	2	5	7
Depreciation	(1)	(1)	(2)	(1)
Other, net	1	(1)		(1)
Consolidated income tax expense	$ 9	$ 13	$ 35	$ 51
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	4.20%	5.60%	5.60%	5.40%
Depreciation, percentage	(4.20%)	(2.80%)	(2.20%)	(0.80%)
Other, net, percentage	2.50%	(1.70%)	0.50%	(0.40%)
Consolidated income tax expense related to continuing operations, percentage	37.50%	36.10%	38.90%	39.20%
Atlantic City Electric Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 7	$ 13	$ 15	$ 22
State income taxes, net of Federal effect	1	2	3	4
Depreciation	(1)		(2)	(1)
Other, net		(1)		(1)
Consolidated income tax expense	$ 7	$ 14	$ 16	$ 24
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	4.80%	5.40%	6.80%	6.30%
Depreciation, percentage	(4.80%)		(4.50%)	(1.60%)
Other, net, percentage	(1.70%)	(2.60%)	(0.90%)	(1.60%)
Consolidated income tax expense related to continuing operations, percentage	33.30%	37.80%	36.40%	38.10%