Regulatory Matters - Additional Information (Detail) (USD $)	1 Months Ended			9 Months Ended						1 Months Ended							9 Months Ended	12 Months Ended				1 Months Ended				9 Months Ended	1 Months Ended		12 Months Ended		1 Months Ended			1 Months Ended				1 Months Ended		1 Months Ended											9 Months Ended	12 Months Ended		1 Months Ended			1 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended	1 Months Ended			1 Months Ended				1 Months Ended					9 Months Ended			1 Months Ended	9 Months Ended	1 Months Ended			9 Months Ended	1 Months Ended
	Aug. 31, 2013	May 31, 2013	Dec. 31, 2012	Sep. 30, 2013	Apr. 30, 2012 MW	Sep. 30, 2013 Minimum [Member]	Apr. 30, 2012 Minimum [Member] MW	Sep. 30, 2013 Maximum [Member]	Apr. 30, 2012 Maximum [Member] MW	May 31, 2013 District of Columbia [Member] DC Undergrounding Task Force [Member]	May 13, 2013 District of Columbia [Member] DC Undergrounding Task Force [Member] Districts	May 31, 2013 Potomac Electric Power Co [Member]	Mar. 08, 2013 Potomac Electric Power Co [Member]	Nov. 30, 2012 Potomac Electric Power Co [Member]	Jul. 31, 2012 Potomac Electric Power Co [Member]	Nov. 01, 2008 Potomac Electric Power Co [Member]	Sep. 30, 2013 Potomac Electric Power Co [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]	Feb. 28, 2013 Potomac Electric Power Co [Member]	Apr. 30, 2012 Potomac Electric Power Co [Member] MW	Nov. 01, 2008 Potomac Electric Power Co [Member] Phi [Member]	Jul. 12, 2013 Potomac Electric Power Co [Member] Subsequent Event [Member]	Nov. 01, 2008 Potomac Electric Power Co [Member] Non Recoverable Rate [Member]	Feb. 27, 2013 Potomac Electric Power Co [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Sep. 30, 2013 Potomac Electric Power Co [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Nov. 30, 2012 Potomac Electric Power Co [Member] Expected [Member]	Jul. 31, 2012 Potomac Electric Power Co [Member] Expected [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member] Expected [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member] Actual [Member]	Feb. 28, 2013 Potomac Electric Power Co [Member] Minimum [Member]	Sep. 30, 2013 Potomac Electric Power Co [Member] Minimum [Member]	Apr. 30, 2012 Potomac Electric Power Co [Member] Minimum [Member] MW	Feb. 27, 2013 Potomac Electric Power Co [Member] Minimum [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Feb. 28, 2013 Potomac Electric Power Co [Member] Maximum [Member]	Sep. 30, 2013 Potomac Electric Power Co [Member] Maximum [Member]	Apr. 30, 2012 Potomac Electric Power Co [Member] Maximum [Member] MW	Feb. 27, 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]	May 13, 2013 Potomac Electric Power Co [Member] District of Columbia [Member] Districts	Aug. 27, 2013 Delmarva Power & Light Co/De [Member]	Aug. 28, 2013 Delmarva Power & Light Co/De [Member]	Aug. 30, 2013 Delmarva Power & Light Co/De [Member]	Jul. 31, 2013 Delmarva Power & Light Co/De [Member]	Jun. 30, 2013 Delmarva Power & Light Co/De [Member]	Mar. 29, 2013 Delmarva Power & Light Co/De [Member]	Mar. 31, 2013 Delmarva Power & Light Co/De [Member]	Feb. 28, 2013 Delmarva Power & Light Co/De [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member]	Nov. 01, 2008 Delmarva Power & Light Co/De [Member]	Sep. 30, 2013 Delmarva Power & Light Co/De [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member]	Apr. 30, 2012 Delmarva Power & Light Co/De [Member] MW	Oct. 02, 2013 Delmarva Power & Light Co/De [Member] Subsequent Event [Member]	Oct. 08, 2013 Delmarva Power & Light Co/De [Member] Subsequent Event [Member]	Sep. 30, 2013 Delmarva Power & Light Co/De [Member] Advanced Metering Infrastructure Costs [Member]	Feb. 27, 2013 Delmarva Power & Light Co/De [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Sep. 30, 2013 Delmarva Power & Light Co/De [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Mar. 29, 2013 Delmarva Power & Light Co/De [Member] Expected [Member]	Sep. 30, 2013 Delmarva Power & Light Co/De [Member] Expected [Member]	Feb. 28, 2013 Delmarva Power & Light Co/De [Member] Minimum [Member]	Sep. 30, 2013 Delmarva Power & Light Co/De [Member] Minimum [Member]	Apr. 30, 2012 Delmarva Power & Light Co/De [Member] Minimum [Member] MW	Feb. 27, 2013 Delmarva Power & Light Co/De [Member] Minimum [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Feb. 28, 2013 Delmarva Power & Light Co/De [Member] Maximum [Member]	Sep. 30, 2013 Delmarva Power & Light Co/De [Member] Maximum [Member]	Apr. 30, 2012 Delmarva Power & Light Co/De [Member] Maximum [Member] MW	Feb. 27, 2013 Delmarva Power & Light Co/De [Member] Maximum [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Mar. 29, 2013 Delmarva Power & Light Co/De [Member] Maximum [Member] Expected [Member]	Mar. 31, 2013 Atlantic City Electric Co [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member]	Feb. 28, 2012 Atlantic City Electric Co [Member]	Sep. 30, 2013 Atlantic City Electric Co [Member]	Jun. 30, 2012 Atlantic City Electric Co [Member]	May 31, 2012 Atlantic City Electric Co [Member]	Jun. 30, 2013 Atlantic City Electric Co [Member] New Jersey Settlement [Member]	Sep. 30, 2013 Atlantic City Electric Co [Member] New Jersey Settlement [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Distribution Rate [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member] Recovery Charges [Member]	Feb. 27, 2013 Atlantic City Electric Co [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Sep. 30, 2013 Atlantic City Electric Co [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Feb. 27, 2013 Atlantic City Electric Co [Member] Minimum [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	Feb. 27, 2013 Atlantic City Electric Co [Member] Maximum [Member] Federal Energy Regulatory Commission Return on Equity Complaint [Member]	May 31, 2013 PHI [Member]
Regulatory Matters [Line Items]
Effect of proposed change on gas cost rate	5.50%																																									5.50%
Distribution base rate													$ 44,100,000	$ 60,800,000	$ 18,100,000												$ 60,800,000	$ 18,100,000	$ 68,400,000	$ 66,200,000																$ 22,800,000	$ 39,000,000		$ 12,000,000					$ 56,000,000
Return on equity, percentage													10.25%	10.25%	9.31%		10.80%		10.75%				9.36%		8.70%	11.30%					10.80%			6.78%	12.80%			10.33%								10.25%	10.25%		10.25%		10.80%						8.70%	11.30%			10.80%			6.78%	12.80%			10.33%			10.25%		10.80%			9.75%	9.75%			8.70%	11.30%	6.78%	10.33%
Rate increase implemented																																													2,500,000			2,500,000
Rate additional increase implemented																																																							25,100,000
Interim rates in effect to refund																																													27,600,000										27,600,000
Increased distribution base period																																																						4 years
Proposed rate of equity																																																						9.75%
Estimated rate of equity																																																						7.41%
Estimated rate of equity																																																						8.80%
Estimated rate of equity																																																						9.75%
Estimated rate of equity																																																						9.75%
Customer refundable fees																																																						500,000
Rate additional increase implemented																																												8,000,000
Rate additional increase implemented																							27,900,000																		6,800,000		15,000,000
Return on equity																							9.36%																		9.75%
Base rate over a period																																																								2 years
Portion of advanced metering infrastructure in percentage																																																								50.00%
Surcharge for recovery of costs																											192,000,000																																10,200,000	10,200,000
Proposed incentive for meeting enhanced reliability goals																											1,000,000																																500,000										500,000
Credit to customers																																							1,000,000																														500,000
ROE for calculating the AFUDC and regulatory asset carrying costs																																											9.81%
Deferred operation and maintenance expenses																							23,600,000																				6,000,000
Period of amortized expenses																							5 years																				5 years
Charges on cost of recovery																							24,000,000																				4,200,000																											67,900,000			67,900,000
Annual depreciation and amortization expenses																	27,300,000		27,300,000
Requested rate change, net																																																																							70,400,000					25,500,000		72,100,000	1,700,000
Requested rate change, net																																																																													25,500,000
Distribution revenue increase																																																																												70,000,000	70,000,000
Capital costs																																																																												44,200,000	44,200,000
Maintenance expenses																																																																												25,800,000	25,800,000
Projected deferred under-recovered balance related to non-utility generators																																																																								113,800,000		127,000,000	113,800,000
Overall annual rate increase of the projected deferred under-recovered balance related to non-utility generators																																																																						52,200,000		55,300,000
Projected deferred under recovered balance																																																																						40,800,000			40,800,000
Additional projected deferred under recovered balance																																																																						32,900,000			32,900,000
Amortization period																																																																						4 years			4 years
Contributions received by ACE																																																																									11,000,000
New power plant output					661		650		700												661												650				700																661										650				700
Aggregate derivative liability				14,000,000		55,000,000		70,000,000																								40,000,000				50,000,000																										15,000,000				20,000,000							14,000,000
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
Number of District of Columbia's outage-prone power lines to bury											60																													60
Congressional review period											30 days																													30 days
Recovery			88,000,000
Recovery period			5 years															5 years																																		5 years
Basis point																	50.00%			200.00%																												200.00%			50.00%																						50.00%
Denied recovery amount																1,000,000						2,000,000																												1,000,000
Recovery percentage																	50.00%							50.00%																										50.00%
Abandoned costs				71,000,000													39,000,000																																		32,000,000
Net of amortization				10,000,000													5,000,000
Abandoned cost original value				88,000,000													50,000,000																																		38,000,000
Disallowed costs written off				2,000,000													1,000,000																																		1,000,000
Abandoned costs transferred to inventories				5,000,000													5,000,000
Amount sought to be recovered																		50,000,000
Capital expenditure																																																				38,000,000
Amortization expenses																																																			$ 5,000,000