Significant Accounting Policies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended								12 Months Ended	3 Months Ended													12 Months Ended														1 Months Ended	3 Months Ended												12 Months Ended					12 Months Ended				12 Months Ended										1 Months Ended	12 Months Ended			3 Months Ended													12 Months Ended						12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2012 Asset Removal Costs [Member]	Dec. 31, 2011 Asset Removal Costs [Member]	Dec. 31, 2012 Derecho [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member]	Sep. 30, 2012 Atlantic City Electric Co [Member]	Jun. 30, 2012 Atlantic City Electric Co [Member]	Mar. 31, 2012 Atlantic City Electric Co [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member]		Sep. 30, 2011 Atlantic City Electric Co [Member]		Jun. 30, 2011 Atlantic City Electric Co [Member]		Mar. 31, 2011 Atlantic City Electric Co [Member]		Dec. 31, 2010 Atlantic City Electric Co [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Y	Dec. 31, 2011 Atlantic City Electric Co [Member]		Dec. 31, 2010 Atlantic City Electric Co [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Minimum [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member] Minimum [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member] Minimum [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Derecho [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Non-Utility Generators [Member] MW	Dec. 31, 2011 Atlantic City Electric Co [Member] Non-Utility Generators [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member] Non-Utility Generators [Member]	Jul. 31, 2012 Delmarva Power & Light Co/De [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Delmarva Power & Light Co/De [Member]	Jun. 30, 2012 Delmarva Power & Light Co/De [Member]	Mar. 31, 2012 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member]		Sep. 30, 2011 Delmarva Power & Light Co/De [Member]		Jun. 30, 2011 Delmarva Power & Light Co/De [Member]		Mar. 31, 2011 Delmarva Power & Light Co/De [Member]		Dec. 31, 2012 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member]		Dec. 31, 2010 Delmarva Power & Light Co/De [Member]	Oct. 31, 2011 Delmarva Power & Light Co/De [Member] MW	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Energy Efficiency Programs [Member] MW	Jun. 30, 2012 Delmarva Power & Light Co/De [Member] Energy Efficiency Programs [Member] MW	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Asset Removal Costs [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member] Asset Removal Costs [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2010 Delmarva Power & Light Co/De [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Derecho [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Wind PPA [Member] MW	Dec. 31, 2011 Delmarva Power & Light Co/De [Member] Wind PPA [Member]	Dec. 31, 2010 Delmarva Power & Light Co/De [Member] Wind PPA [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Wind PPA [Member] Wind Facility One [Member] MW	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Wind PPA [Member] Wind Facility Two [Member] MW	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Wind PPA [Member] Wind Facility Three [Member] MW	Oct. 31, 2011 Delmarva Power & Light Co/De [Member] Solar PPA [Member] Y	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Solar PPA [Member] Y MW	Dec. 31, 2011 Delmarva Power & Light Co/De [Member] Solar PPA [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member] Land-Based Wind PPA [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member]	Sep. 30, 2012 Potomac Electric Power Co [Member]	Jun. 30, 2012 Potomac Electric Power Co [Member]	Mar. 31, 2012 Potomac Electric Power Co [Member]		Dec. 31, 2011 Potomac Electric Power Co [Member]		Sep. 30, 2011 Potomac Electric Power Co [Member]		Jun. 30, 2011 Potomac Electric Power Co [Member]		Mar. 31, 2011 Potomac Electric Power Co [Member]		Dec. 31, 2012 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]		Dec. 31, 2010 Potomac Electric Power Co [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member] Asset Removal Costs [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member] Asset Removal Costs [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2010 Potomac Electric Power Co [Member] Electric Transmission and Distribution [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member] Derecho [Member]
Significant Accounting Policies [Line Items]
Number of purchase power agreements																											3																																															1		3
Megawatts received from power purchase agreements (PPAs)																																						459																													128							10
Net power purchases with non-utility generators																																						$ 206	$ 218	$ 292
Purchased energy					2,476	3,453	4,632																															201	206	270																											27	18	12
Energy purchase maximum to be purchased, MW																																																																						50	40	38
Term of agreement, years																											15																																														21	20
Obligated purchase amount of energy produced at the facility																																																																										70.00%
Solar energy purchases																																																																										2	1
Power of fuel cell facility, MW																																																										30	27	3
Increments to power of fuel cell facility, MW																																																											5
Amount billed to distribution customers																																																											4
Equity ownership percentage														100.00%													100.00%
Number of Standard Offer Capacity Agreements																											3
Incremental storm costs			33.0										138.0														20.0										72.0				4.6																									17.0																								9.9									49.0
Other operation and maintenance					911	914	884						66														239	226		204							27																	260	239		255									11																								403	420		354						28
Capital Expenditure													72																								45																													6																																	21
Regulatory assets			2,614		2,614	2,196							56	694				662									694	662														288				227								288	227											6											487					299								487	299								23
Other operation and maintenance expense				10																																																														5																																	5
Self-insurance reserves reduced						4																					1	1																										1	2																																			1	1
Income tax (expense) benefit after-tax		15												(3)	13	9	(1)	(3)	[1]	17	[1]	14	[1]	5	[1]		18	33	[1]													10	11	9	14	10	[2]	10	[2]	5	[2]	17	[2]	44	42	[2]																					10	35	8	(5)	[3]	4	[4]	23	[4]	2	[4]	7	[4]	48	36	[4]
Additional income tax (expense) benefit after-tax	17				(3)										3							1				1		3															1					1							4																								4							5					(1)
Unbilled revenue			182		182	179								39				41									39	41														62				56								62	56																						81					82								81	82
Taxes included in gross revenues					356	378	362																				15	22		23																								15	18		17																																	324	338		322
Awards received from the U.S. DOE																														19																																																															149
AFUDC for borrowed funds					7	11	8																				1	2		2																								2	1		2																																	4	8		4
AFUDC for equity component					14	15	10																				3				3	1	1																					3	3		4																																	8	12		6
Regulatory Liabilities			501		501	526					324	388		102				60									102	60														258				297								258	297						202	244															141					169								141	169			122	144
Adjustment						31	1																																															1
Increase in revenue and pre-tax earnings						2
Reclassification adjustment																														6																									11																																						5
Increase of deferred electric service cost																											3
Asset retirement obligation accrual		4
Asset retirement obligation amounts that should have been recorded in period								1	1	1
Reclassification adjustment relating to uncertain tax positions						2																						1																																																															1
Reclassification adjustment relating to continuing operations							(5)
Annual Depreciation Rate																																		3.00%	3.00%	2.80%																											2.70%	2.80%	2.80%																															2.50%	2.60%	2.60%
Retained earnings			1,109		1,109	1,072								200				200									200	200														578				505								578	505																						888					797								888	797
Regulatory Liabilities																																										$ 4				$ 15								$ 4	$ 15

[1] Includes tax expense of $1 million (after-tax) associated with interest related to federal tax liabilities in the second quarter and an additional tax expense of $3 million (after-tax) resulting from a recalculation of interest on uncertain tax positions for open tax years in the third quarter.

[2] Includes tax benefits of $4 million (after-tax) associated with an interest benefit related to federal tax liabilities in the second quarter and an additional tax expense of $1 million (after-tax) resulting from a recalculation of interest on uncertain tax positions for open tax years in the third quarter.

[3] Includes tax benefits of $10 million (after-tax), primarily related to the settlement of an uncertain tax position with the IRS related to the methodology used historically to calculate deductible mixed service costs and the expiration of the statute of limitations associated with an uncertain tax position.

[4] Includes tax benefits of $5 million (after-tax) associated with an interest benefit related to federal tax liabilities and an additional tax benefit of $4 million (after-tax) related to the filing of amended state tax returns, each recorded in the second quarter.