Income Taxes - Reconciliation of Consolidated Income Tax Expense from Continuing Operations (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate									$ 133,000,000	$ 150,000,000	$ 112,000,000
State income taxes, net of federal effect									23,000,000	27,000,000	19,000,000
Asset removal costs									(12,000,000)	(14,000,000)	(11,000,000)
Change in estimates and interest related to uncertain and effectively settled tax positions										56,000,000	(8,000,000)
Establishment of valuation allowances related to deferred tax assets										101,000,000
Merger related costs									7,000,000
Other, net									(13,000,000)	(1,000,000)	(9,000,000)
Consolidated Income Tax Expense Related to Continuing Operations	13,000,000	34,000,000	45,000,000	46,000,000	39,000,000	65,000,000	30,000,000	185,000,000	138,000,000	319,000,000	103,000,000
Income tax at Federal statutory rate, percentage									35.00%	35.00%	35.00%
State income taxes, net of federal effect, percentage									6.10%	6.30%	6.00%
Asset removal costs, percentage									(3.20%)	(3.30%)	(3.40%)
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage										13.10%	(2.60%)
Establishment of valuation allowances related to deferred tax assets, percentage										23.50%
Merger related costs, percentage									1.80%
Other, net, percentage									(3.40%)	(0.20%)	(2.90%)
Consolidated Income Tax Expense Related to Continuing Operations, percentage									36.30%	74.40%	32.10%
Potomac Electric Power Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate									92,000,000	80,000,000	61,000,000
State income taxes, net of federal effect									15,000,000	13,000,000	10,000,000
Asset removal costs									(12,000,000)	(14,000,000)	(11,000,000)
Change in estimates and interest related to uncertain and effectively settled tax positions									(1,000,000)	(3,000,000)	(11,000,000)
Establishment of valuation allowances related to deferred tax assets									0	0
Other, net									(2,000,000)	3,000,000	(1,000,000)
Consolidated Income Tax Expense Related to Continuing Operations	10,000,000	38,000,000	28,000,000	16,000,000	17,000,000	40,000,000	20,000,000	2,000,000	92,000,000	79,000,000	48,000,000
Income tax at Federal statutory rate, percentage									35.00%	35.00%	35.00%
State income taxes, net of federal effect, percentage									5.70%	5.70%	5.70%
Asset removal costs, percentage									(4.60%)	(6.10%)	(6.30%)
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage									(0.40%)	(1.30%)	(6.30%)
Other, net, percentage									(0.70%)	1.20%	(0.50%)
Consolidated Income Tax Expense Related to Continuing Operations, percentage									35.00%	34.50%	27.60%
Delmarva Power & Light Co/De [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate									59,000,000	51,000,000	41,000,000
State income taxes, net of federal effect									9,000,000	8,000,000	6,000,000
Establishment of valuation allowances related to deferred tax assets									0	0
Other, net									(3,000,000)	(3,000,000)	(3,000,000)
Consolidated Income Tax Expense Related to Continuing Operations	14,000,000	13,000,000	13,000,000	25,000,000	17,000,000	14,000,000	9,000,000	16,000,000	65,000,000	56,000,000	44,000,000
Income tax at Federal statutory rate, percentage									35.00%	35.00%	35.00%
State income taxes, net of federal effect, percentage									5.30%	5.50%	5.10%
Other, net, percentage									(1.80%)	(1.90%)	(2.50%)
Consolidated Income Tax Expense Related to Continuing Operations, percentage									38.50%	38.60%	37.60%
Atlantic City Electric Co [Member]
Income Tax Rate Reconciliation [Line Items]
Income tax at Federal statutory rate									26,000,000	24,000,000	19,000,000
State income taxes, net of federal effect									4,000,000	5,000,000	3,000,000
Change in estimates and interest related to uncertain and effectively settled tax positions									(1,000,000)	(9,000,000)	(1,000,000)
Establishment of valuation allowances related to deferred tax assets									0	0
Plant basis adjustments										(2,000,000)	(1,000,000)
Investment tax credit amortization										(1,000,000)	(1,000,000)
Other, net									(1,000,000)	2,000,000	(1,000,000)
Consolidated Income Tax Expense Related to Continuing Operations	$ 4,000,000	$ 14,000,000	$ 4,000,000	$ 6,000,000	$ 5,000,000	$ 13,000,000	$ 4,000,000	$ (3,000,000)	$ 28,000,000	$ 19,000,000	$ 18,000,000
Income tax at Federal statutory rate, percentage									35.00%	35.00%	35.00%
State income taxes, net of federal effect, percentage									5.50%	7.20%	5.70%
Change in estimates and interest related to uncertain and effectively settled tax positions, percentage									(1.40%)	(13.00%)	(1.90%)
Plant basis adjustments, percentage										(2.90%)	(1.90%)
Investment tax credit amortization, percentage										(1.40%)	(1.90%)
Other, net, percentage									(0.70%)	2.60%	(1.00%)
Consolidated Income Tax Expense Related to Continuing Operations, percentage									38.40%	27.50%	34.00%