Income Taxes - Reconciliation of Consolidated Income Tax Expense (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 28	$ 34	$ 57	$ 77
State income taxes, net of Federal effect	4	7	10	15
Asset removal costs	(5)	(2)	(8)	(5)
Energy efficiency-related tax deductions			(4)
Merger-related costs	1	7	3	7
Other, net	(1)	(1)	(1)	(3)
Consolidated income tax expense	$ 27	$ 45	$ 57	$ 91
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	5.00%	7.10%	6.10%	6.80%
Asset removal costs, percentage	(6.30%)	(2.00%)	(4.90%)	(2.30%)
Energy efficiency-related tax deductions, percentage			(2.50%)
Merger related costs, percentage	1.20%	7.10%	1.80%	3.20%
Other, net, percentage	(1.10%)	(1.30%)	(0.50%)	(1.10%)
Consolidated income tax expense related to continuing operations, percentage	33.80%	45.90%	35.00%	41.60%
Potomac Electric Power Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 21	$ 26	$ 34	$ 43
State income taxes, net of Federal effect	3	4	5	7
Asset removal costs	(5)	(2)	(8)	(5)
Change in estimates and interest related to uncertain and effectively settled tax positions				(1)
Other, net	(1)		(1)
Consolidated income tax expense	$ 18	$ 28	$ 30	$ 44
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	5.00%	5.40%	5.10%	5.70%
Asset removal costs, percentage	(8.30%)	(2.70%)	(8.20%)	(4.10%)
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage				(0.80%)
Other, net, percentage	(1.70%)	0.10%	(1.30%)	0.30%
Consolidated income tax expense related to continuing operations, percentage	30.00%	37.80%	30.60%	36.10%
Delmarva Power & Light Co/De [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 5	$ 11	$ 23	$ 33
State income taxes, net of Federal effect	1	2	4	5
Depreciation	(1)		(1)	(1)
Other, net				1
Consolidated income tax expense	$ 5	$ 13	$ 26	$ 38
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	7.70%	6.30%	6.10%	5.30%
Depreciation, percentage	(7.70%)		(1.50%)	(1.10%)
Other, net, percentage	3.50%	(0.70%)	(0.20%)	1.20%
Consolidated income tax expense related to continuing operations, percentage	38.50%	40.60%	39.40%	40.40%
Atlantic City Electric Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate	$ 6	$ 3	$ 8	$ 9
State income taxes, net of Federal effect	1	1	1	2
Depreciation				(1)
Consolidated income tax expense	$ 7	$ 4	$ 9	$ 10
Income tax at Federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of Federal effect, percentage	5.90%	10.00%	4.30%	7.70%
Depreciation, percentage				(3.80%)
Other, net, percentage	0.30%	(5.00%)	(0.20%)	(0.40%)
Consolidated income tax expense related to continuing operations, percentage	41.20%	40.00%	39.10%	38.50%