Regulatory Matters - Additional Information (Detail) (USD $)	0 Months Ended							1 Months Ended					9 Months Ended	12 Months Ended		0 Months Ended			1 Months Ended		9 Months Ended			9 Months Ended		0 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		0 Months Ended		0 Months Ended								1 Months Ended				9 Months Ended	12 Months Ended			0 Months Ended	9 Months Ended	0 Months Ended					1 Months Ended	9 Months Ended	1 Months Ended		0 Months Ended		0 Months Ended			0 Months Ended										1 Months Ended	9 Months Ended	12 Months Ended				0 Months Ended		9 Months Ended				0 Months Ended		9 Months Ended		0 Months Ended		0 Months Ended
	Aug. 29, 2014	Aug. 20, 2014	Aug. 05, 2014	Apr. 15, 2014	Mar. 26, 2014	Sep. 20, 2013	Aug. 28, 2013	Mar. 31, 2014	Feb. 28, 2014	Oct. 31, 2013	Jul. 31, 2012	Dec. 31, 2011	Sep. 30, 2014 mi	Dec. 31, 2013	Apr. 30, 2012 MW	Aug. 25, 2014 Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Jul. 02, 2014 Expected [Member]	Dec. 04, 2013 Expected [Member]	Dec. 31, 2011 Expected [Member]	Dec. 31, 2011 Actual [Member]	Sep. 30, 2014 Advanced Metering Infrastructure Costs [Member]	Jul. 11, 2014 Advanced Metering Infrastructure Costs [Member]	Sep. 30, 2014 Advanced Metering Infrastructure Costs [Member] April 1, 2015 [Member]	Sep. 30, 2014 2014 [Member]	Sep. 30, 2014 2015 [Member]	Oct. 22, 2014 Subsequent Event [Member]	Sep. 30, 2014 Delaware [Member]	Oct. 31, 2013 District of Columbia [Member] DC Undergrounding Task Force [Member]	Apr. 30, 2012 Minimum [Member] MW	Apr. 30, 2012 Maximum [Member] MW	Aug. 20, 2014 Atlantic City Electric Co [Member]	Apr. 16, 2014 Atlantic City Electric Co [Member]	Mar. 14, 2014 Atlantic City Electric Co [Member]	Mar. 03, 2014 Atlantic City Electric Co [Member]	May 31, 2014 Atlantic City Electric Co [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member]	Feb. 28, 2013 Atlantic City Electric Co [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Oct. 22, 2014 Atlantic City Electric Co [Member] Subsequent Event [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member] Minimum [Member]	Feb. 28, 2013 Atlantic City Electric Co [Member] Minimum [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Sep. 30, 2014 Atlantic City Electric Co [Member] Maximum [Member]	Feb. 28, 2013 Atlantic City Electric Co [Member] Maximum [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Apr. 15, 2014 Potomac Electric Power Co [Member]	Mar. 26, 2014 Potomac Electric Power Co [Member]	Dec. 03, 2013 Potomac Electric Power Co [Member]	Jul. 12, 2013 Potomac Electric Power Co [Member]	Mar. 08, 2013 Potomac Electric Power Co [Member]	Nov. 30, 2012 Potomac Electric Power Co [Member]	Aug. 24, 2012 Potomac Electric Power Co [Member] mi	Feb. 28, 2014 Potomac Electric Power Co [Member]	Oct. 31, 2013 Potomac Electric Power Co [Member]	Jul. 31, 2012 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]	Sep. 30, 2014 Potomac Electric Power Co [Member]	Dec. 31, 2013 Potomac Electric Power Co [Member]	Jul. 12, 2013 Potomac Electric Power Co [Member]	Apr. 30, 2012 Potomac Electric Power Co [Member] MW	Feb. 28, 2013 Potomac Electric Power Co [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Sep. 30, 2014 Potomac Electric Power Co [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Dec. 03, 2013 Potomac Electric Power Co [Member] Adjusted [Member]	Jul. 02, 2014 Potomac Electric Power Co [Member] Expected [Member]	Mar. 26, 2014 Potomac Electric Power Co [Member] Expected [Member]	Dec. 04, 2013 Potomac Electric Power Co [Member] Expected [Member]	Nov. 30, 2012 Potomac Electric Power Co [Member] Expected [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member] Actual [Member]	Sep. 30, 2014 Potomac Electric Power Co [Member] District of Columbia [Member]	Oct. 31, 2013 Potomac Electric Power Co [Member] District of Columbia [Member] DC Undergrounding Task Force [Member]	Apr. 30, 2012 Potomac Electric Power Co [Member] Minimum [Member] MW	Feb. 28, 2013 Potomac Electric Power Co [Member] Minimum [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Apr. 30, 2012 Potomac Electric Power Co [Member] Maximum [Member] MW	Feb. 28, 2013 Potomac Electric Power Co [Member] Maximum [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Nov. 30, 2012 Potomac Electric Power Co [Member] Maximum [Member] Expected [Member]	Nov. 30, 2012 Potomac Electric Power Co [Member] Maximum [Member] Expected [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member]	Aug. 29, 2014 Delmarva Power & Light Co/De [Member]	Aug. 19, 2014 Delmarva Power & Light Co/De [Member]	Aug. 05, 2014 Delmarva Power & Light Co/De [Member]	Oct. 08, 2013 Delmarva Power & Light Co/De [Member]	Oct. 02, 2013 Delmarva Power & Light Co/De [Member]	Sep. 20, 2013 Delmarva Power & Light Co/De [Member]	Aug. 28, 2013 Delmarva Power & Light Co/De [Member]	Jun. 01, 2013 Delmarva Power & Light Co/De [Member]	Mar. 22, 2013 Delmarva Power & Light Co/De [Member]	Feb. 28, 2014 Delmarva Power & Light Co/De [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] mi	Dec. 31, 2013 Delmarva Power & Light Co/De [Member]	Oct. 08, 2013 Delmarva Power & Light Co/De [Member]	Oct. 02, 2013 Delmarva Power & Light Co/De [Member]	Apr. 30, 2012 Delmarva Power & Light Co/De [Member] MW	Aug. 25, 2014 Delmarva Power & Light Co/De [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Feb. 28, 2013 Delmarva Power & Light Co/De [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] Advanced Metering Infrastructure Costs [Member]	Jul. 11, 2014 Delmarva Power & Light Co/De [Member] Advanced Metering Infrastructure Costs [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] Advanced Metering Infrastructure Costs [Member] April 1, 2015 [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] 2014 [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] 2015 [Member]	Sep. 30, 2014 Delmarva Power & Light Co/De [Member] Delaware [Member]	Apr. 30, 2012 Delmarva Power & Light Co/De [Member] Minimum [Member] MW	Feb. 28, 2013 Delmarva Power & Light Co/De [Member] Minimum [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]	Apr. 30, 2012 Delmarva Power & Light Co/De [Member] Maximum [Member] MW	Feb. 28, 2013 Delmarva Power & Light Co/De [Member] Maximum [Member] Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate change		$ 19,000,000			$ 23,400,000	$ 39,000,000		$ 61,700,000			$ 18,100,000						$ 8,750,000	$ 43,300,000	$ 68,400,000	$ 66,200,000											$ 19,000,000		$ 61,700,000													$ 44,800,000		$ 52,100,000					$ 18,100,000	$ 68,400,000							$ 44,800,000	$ 8,750,000	$ 23,400,000	$ 43,300,000	$ 60,800,000	$ 66,200,000												$ 15,100,000		$ 56,000,000	$ 39,000,000			$ 42,000,000
Return on equity, percentage		9.75%			9.40%			10.25%			9.31%	10.75%					9.62%	10.25%													9.75%		10.25%			10.80%	8.70%	11.30%			6.78%		10.33%		9.40%	10.25%	9.36%	10.25%	10.25%				9.31%	10.75%	10.80%				8.70%	11.30%		9.62%		10.25%						6.78%		10.33%						9.70%	9.70%					10.25%		10.80%						8.70%	11.30%								6.78%		10.33%
Interim rate increase implemented																																																																																			2,500,000
Additional interim rate increase implemented			15,100,000																																																																												25,100,000
Interim rates in effect subject to refund																																																																																								27,600,000
Increased distribution base period																																																																																4 years
Estimated return on equity, year one																																																																																7.41%
Estimated return on equity, year two																																																																																8.80%
Estimated return on equity, year three																																																																																9.75%
Estimated return on equity, year four																																																																																9.75%
Customer refundable fees																																																																																									500,000
Base rates period																					2 years																																																																									2 years
Related portion of advanced metering infrastructure																						50.00%	50.00%																																																																								50.00%	50.00%
Effect of proposed change on Gas Cost Rate	(7.40%)						(5.50%)																																																																					(7.40%)						(5.50%)
Surcharge for recovery of costs																																																																	192,000,000
Proposed incentive for meeting enhanced reliability goals																																																																	1,000,000								1,000,000
Credit to customers																																																																										1,000,000
Requested rate change																																															27,900,000																				23,400,000
Deferred operation and maintenance expenses																																																									23,600,000
Period of amortized expenses																																															5 years
Charges on cost of recovery																																															24,000,000
Adjusted rate change				37,400,000																																								37,400,000
Overall annual rate decrease as net impact of adjusting charges																																41,100,000		24,500,000	41,100,000
Contributions received by ACE																																				11,000,000				1,000,000		1,000,000
Consolidated tax adjustment calculation review period																										5 years													5 years
Percentage of revenue requirement related to consolidated tax adjustment calculation																										25.00%													25.00%
One-time reduction on settlement																225,000																																																																											225,000
One-time payment on settlement																258,500																																																																											258,500
Basis-point																																				0.50%																			0.50%																															0.50%
New power plant output															661														650	700																												661											650		700																			661										650		700
Undergrounding project cost										1,000,000,000																																										1,000,000,000
Bear estimated complete project										500,000,000																																										500,000,000
Underground project cost																												375,000,000																																								375,000,000
Abandonment costs sought									88,000,000				80,500,000	82,000,000																																					50,000,000				43,900,000	45,000,000																													38,000,000	36,600,000	37,000,000
Recovery period													3 years																																										3 years																														3 years
Estimated fair value of real property acquired									8,000,000																																										2,000,000																																		6,000,000
Length transmission line													152																																					152																																				152
Abandonment costs													39,000,000
Expected pre-tax income related to abandonment costs																								3,000,000	1,000,000																																																																								3,000,000	1,000,000
Land held for future use in utility operations													8,000,000																																										2,000,000																															6,000,000
Number of days to grant approval																											120 days																																																																								120 days
Period to issue decision													180 days																							180 days																			180 days																						180 days									180 days
Merger agreement													Following informal discussions with the DOJ, effective as of September 5, 2014, Exelon withdrew its Notification and Report Form and refiled it on September 9, 2014, which restarted the waiting period required by the HSR Act. On October 9, 2014, each of Pepco Holdings and Exelon received a request for additional information and documentary material from the DOJ, which has the effect of extending the DOJ review period until 30 days after each of Pepco Holdings and Exelon has certified that it has substantially complied with the request.
Return on equity, percentage																																															9.36%																				9.40%
Regulatory asset related to MAPP abandonment costs																																																							$ 23,000,000																				$ 16,000,000