Income Taxes - Reconciliation of Consolidated Income Tax Expense from Continuing Operations (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Tax Rate Reconciliation [Line Items]
Income tax at federal statutory rate	$ 40	$ 61	$ 116	$ 116
State income taxes, net of federal effect	6	11	21	21
Asset removal costs	(4)	(5)	(9)	(11)
Change in estimates and interest related to uncertain and effectively settled tax positions		1		55
Energy efficiency-related tax deductions	(4)		(4)
Establishment of valuation allowances related to deferred tax assets				101
Merger-related costs			7
Other, net	(4)	(3)	(6)	(2)
Consolidated income tax expense related to continuing operations	34	65	125	280
Income tax at federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of federal effect, percentage	5.30%	6.30%	6.30%	6.30%
Asset removal costs, percentage	(3.50%)	(2.90%)	(2.70%)	(3.30%)
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage		0.60%		16.60%
Energy efficiency-related tax deductions, percentage	(3.50%)		(1.20%)
Establishment of valuation allowances related to deferred tax assets, percentage				30.40%
Merger-related costs, percentage			2.10%
Other, net, percentage	(3.20%)	(1.90%)	(1.90%)	(0.70%)
Consolidated income tax expense related to continuing operations, percentage	30.10%	37.10%	37.60%	84.30%
Potomac Electric Power Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at federal statutory rate	37	37	79	66
State income taxes, net of federal effect	6	6	13	11
Asset removal costs	(4)	(5)	(9)	(11)
Change in estimates and interest related to uncertain and effectively settled tax positions				(4)
Other, net	(1)	2	(1)
Consolidated income tax expense related to continuing operations	38	40	82	62
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%	5.70%	5.70%	5.90%
Asset removal costs, percentage	(3.80%)	(4.70%)	(4.00%)	(5.90%)
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage				(2.10%)
Other, net, percentage	(0.70%)	1.70%	(0.60%)	0.10%
Consolidated income tax expense related to continuing operations, percentage	36.20%	37.70%	36.10%	33.00%
Delmarva Power & Light Co/De [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at federal statutory rate	13	13	46	35
State income taxes, net of federal effect	2	2	7	5
Change in estimates and interest related to uncertain and effectively settled tax positions				(1)
Other, net	(2)	(1)	(2)
Consolidated income tax expense related to continuing operations	13	14	51	39
Income tax at federal statutory rate, percentage	35.00%	35.00%	35.00%	35.00%
State income taxes, net of federal effect, percentage	5.60%	5.40%	5.40%	5.00%
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage				(1.00%)
Other, net, percentage	(4.50%)	(2.60%)	(1.20%)
Consolidated income tax expense related to continuing operations, percentage	36.10%	37.80%	39.20%	39.00%
Atlantic City Electric Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at federal statutory rate	13	13	22	19
State income taxes, net of federal effect	2	2	4	4
Change in estimates and interest related to uncertain and effectively settled tax positions				(9)
Other, net	(1)	(2)	(2)
Consolidated income tax expense related to continuing operations	$ 14	$ 13	$ 24	$ 14
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%	5.30%	6.30%	7.30%
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage				(16.40%)
Other, net, percentage	(2.60%)	(6.10%)	(3.20%)	(0.40%)
Consolidated income tax expense related to continuing operations, percentage	37.80%	34.20%	38.10%	25.50%