Regulatory Matters (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended			9 Months Ended					1 Months Ended	0 Months Ended	1 Months Ended						9 Months Ended		0 Months Ended	9 Months Ended	0 Months Ended			9 Months Ended	0 Months Ended	1 Months Ended					9 Months Ended		0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	3 Months Ended	9 Months Ended		1 Months Ended
	Aug. 24, 2012	Aug. 31, 2012	Aug. 31, 2011	Sep. 30, 2012	Apr. 12, 2012 MW	Dec. 31, 2011	Apr. 12, 2012 Maximum [Member] MW	Apr. 12, 2012 Minimum [Member] MW	Aug. 24, 2012 Delmarva Power & Light Co/De [Member]	Dec. 09, 2011 Delmarva Power & Light Co/De [Member]	Aug. 31, 2012 Delmarva Power & Light Co/De [Member]	Aug. 17, 2012 Delmarva Power & Light Co/De [Member]	Jul. 03, 2012 Delmarva Power & Light Co/De [Member]	Jan. 31, 2012 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member]	Aug. 31, 2011 Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Delmarva Power & Light Co/De [Member]	Apr. 12, 2012 Delmarva Power & Light Co/De [Member] MW	Dec. 09, 2011 Delmarva Power & Light Co/De [Member] Expected [Member]	Sep. 30, 2012 Delmarva Power & Light Co/De [Member] Expected [Member]	Dec. 09, 2011 Delmarva Power & Light Co/De [Member] Actual [Member]	Apr. 12, 2012 Delmarva Power & Light Co/De [Member] Maximum [Member] MW	Apr. 12, 2012 Delmarva Power & Light Co/De [Member] Minimum [Member] MW	Sep. 30, 2012 Delmarva Power & Light Co/De [Member] State and local taxes [Member]	Dec. 16, 2011 Potomac Electric Power Co [Member]	Sep. 26, 2012 Potomac Electric Power Co [Member]	Aug. 24, 2012 Potomac Electric Power Co [Member]	Jun. 28, 2011 Potomac Electric Power Co [Member]	Jun. 29, 2011 Potomac Electric Power Co [Member]	Jul. 08, 2011 Potomac Electric Power Co [Member]	Sep. 30, 2012 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]	Dec. 16, 2011 Potomac Electric Power Co [Member] Expected [Member]	Sep. 30, 2012 Potomac Electric Power Co [Member] Expected [Member]	Dec. 16, 2011 Potomac Electric Power Co [Member] Actual [Member]	Sep. 30, 2012 Potomac Electric Power Co [Member] Hurricane [Member]	Aug. 05, 2011 Atlantic City Electric Co [Member]	Mar. 31, 2012 Atlantic City Electric Co [Member]	Sep. 30, 2012 Atlantic City Electric Co [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member]	Oct. 23, 2012 Atlantic City Electric Co [Member] New Jersey Settlement [Member]	Feb. 24, 2012 Atlantic City Electric Co [Member] Maximum [Member]	Oct. 23, 2012 Atlantic City Electric Co [Member] Hurricane [Member] New Jersey Settlement [Member]
Regulatory Matters [Line Items]
Effect of proposed change on gas cost rate		22.30%	5.60%								22.30%					5.60%
Requested rate change												$ 22.0			$ 31.8				$ 25.2	$ 11.3	$ 23.5					$ 24.0				$ 42.0			$ 68.4	$ 18.1	$ 66.2		$ 90.3				$ 44.0
Return on equity, percentage										10.75%		9.75%			10.75%		9.81%								10.75%	9.50%				10.75%	9.31%						10.75%				9.75%
Rate increase implemented													22.3	2.5
State and local taxes collectable as per settlement agreement				2,421.0		2,196.0									227.0		271.0							3.4							455.0	299.0							696.0	662.0
State and local taxes collectable as per settlement agreement, period																								Three year
Recover costs associated with the AMI project																										9				9
Incremental storm restoration costs				27.0													4.6														9.9					2.0			12.0				7.7
Annual depreciation and amortization expenses																	4.1														27.3
Recover in depreciation rates																	4.3											8.8	9.7		18.5
Recovery of requested rate change																															1.6
Final rate increase																															16.5
Requested rate change, net																																					54.6				28.0	74.3
Base rate change related to excess depreciation																																					16				16
Credit rider expected to expire																																					August 31, 2013
Base rate change related to increase in sales and use taxes																																					6.3				2.0
Projected deferred under-recovered balance related to non-utility generators																																						113.8
Overall annual rate increase of the projected deferred under-recovered balance related to non-utility generators																																						55.3
New power plant output					661		700	650										661				700	650
Projected capital expenditure reallocated	205								67																		138
Projected Capital Expenditure Projected Years	5 years								5 years																		5 years
Capital expenditure incurred for terminated Project				101													37														64
Incentive rate of return				12.80%													12.80%														12.80%
Capital expenditure placed in service				11																											11
Capital expenditure reclassified to regulatory Asset				$ 90													$ 37														$ 53