Significant Accounting Policies (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended				9 Months Ended		12 Months Ended	9 Months Ended	1 Months Ended	3 Months Ended			9 Months Ended				9 Months Ended		3 Months Ended			9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended					3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Derecho [Member]	Jun. 30, 2011 Phi [Member]	Sep. 30, 2012 Atlantic City Electric Co [Member]	Sep. 30, 2011 Atlantic City Electric Co [Member]	Jun. 30, 2011 Atlantic City Electric Co [Member]	Sep. 30, 2012 Atlantic City Electric Co [Member]	Sep. 30, 2011 Atlantic City Electric Co [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member] Y	Sep. 30, 2012 Atlantic City Electric Co [Member] Derecho [Member]	Oct. 31, 2011 Delmarva Power & Light Co/De [Member] Y MW	Sep. 30, 2012 Delmarva Power & Light Co/De [Member]	Sep. 30, 2011 Delmarva Power & Light Co/De [Member]	Jun. 30, 2011 Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Delmarva Power & Light Co/De [Member]	Sep. 30, 2011 Delmarva Power & Light Co/De [Member]	Jan. 31, 2013 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	Sep. 30, 2012 Delmarva Power & Light Co/De [Member] Derecho [Member]	Sep. 30, 2011 Pepco Energy Services [Member]	Sep. 30, 2012 Potomac Electric Power Co [Member]	Sep. 30, 2011 Potomac Electric Power Co [Member]	Jun. 30, 2011 Potomac Electric Power Co [Member]	Sep. 30, 2012 Potomac Electric Power Co [Member]	Sep. 30, 2011 Potomac Electric Power Co [Member]	Jun. 30, 2012 Potomac Electric Power Co [Member] Derecho [Member]	Sep. 30, 2012 Pepco Holdings Inc [Member]	Sep. 30, 2012 Non-Utility Generators [Member] Atlantic City Electric Co [Member]	Sep. 30, 2011 Non-Utility Generators [Member] Atlantic City Electric Co [Member]	Sep. 30, 2012 Non-Utility Generators [Member] Atlantic City Electric Co [Member] MW	Sep. 30, 2011 Non-Utility Generators [Member] Atlantic City Electric Co [Member]	Sep. 30, 2012 Wind Ppa [Member] Delmarva Power & Light Co/De [Member]	Sep. 30, 2011 Wind Ppa [Member] Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Wind Ppa [Member] Delmarva Power & Light Co/De [Member] MW	Sep. 30, 2011 Wind Ppa [Member] Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Wind Ppa [Member] Delmarva Power & Light Co/De [Member] Wind Facility One [Member] MW	Sep. 30, 2012 Wind Ppa [Member] Delmarva Power & Light Co/De [Member] Wind Facility Two [Member] MW	Sep. 30, 2012 Wind Ppa [Member] Delmarva Power & Light Co/De [Member] Wind Facility Three [Member] MW	Sep. 30, 2012 Solar Ppa [Member] Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Solar Ppa [Member] Delmarva Power & Light Co/De [Member] Y MW	Sep. 30, 2012 Land-Based Wind PPA [Member] Delmarva Power & Light Co/De [Member]
Significant Accounting Policies [Line Items]
Incremental storm costs			$ 27.0		$ 81.0					$ 12.0			$ 36.0					$ 4.6				$ 3.0					$ 9.9		$ 42.0
Other operation and maintenance	230	239	679	682	40		66	61		178	167		13		65	69		192	181			2		97	111		301	313	25
Capital Expenditure					41								23									1							17
Regulatory assets					35								13									1							21
Other operation and maintenance expense					5																	1							4
Self-insurance reserves reduced						4	1		1	1					1		2	1								1
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																															56	57	156	169
Purchased energy	709	948	1,948	2,759																											53	55	151	159	4	3	20	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																																										1	2
Power of fuel cell facility, mW														30						27	3
Increments to power of fuel cell facility, mW																				5
Equity ownership percentage							100.00%			100.00%
Number of Standard Offer Capacity Agreements												3
Taxes included in gross revenues	110	111	293	302			6	7		13	17				4	4		12	14					100	100		268	271
Operating revenue		5	1	12														1
Increase in revenue and pre-tax earnings																							2
Other operation and maintenance expense																(1)			(9)
Deferred tax Adjustment																														1
Increase in income tax expense								$ 1			$ 1																	$ 1		$ 1