Regulatory Matters - Additional Information (Detail)												1 Months Ended													9 Months Ended	27 Months Ended
	Sep. 11, 2015 USD ($)	Aug. 27, 2015	Jun. 01, 2015 USD ($)	May. 19, 2015 USD ($)	Apr. 17, 2015 USD ($)	Dec. 08, 2014	Dec. 07, 2014	Aug. 27, 2014	Apr. 15, 2014 USD ($)	Sep. 20, 2013 USD ($)	Feb. 28, 2013	Mar. 31, 2015 USD ($)	Oct. 31, 2014	Aug. 31, 2014 USD ($)	Jul. 31, 2014 USD ($)	May. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Oct. 31, 2013 USD ($)	Jul. 31, 2013 USD ($)	Mar. 31, 2013 USD ($)	Feb. 28, 2013	Nov. 30, 2012 USD ($)	Jul. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Apr. 30, 2012 MW
Public Utilities, General Disclosures [Line Items]
Requested rate change										$ 39,000,000
Return on equity, percentage																								10.75%
Approved rate change															$ 8,750,000								$ 18,100,000
Return on equity, percentage															9.62%								9.31%
Effect of proposed change on Gas Cost Rate		26.00%												7.40%
Adjusted rate change									$ 37,400,000
Consolidated tax adjustment calculation review period													5 years
Percentage of revenue requirement related to consolidated tax adjustment calculation													25.00%
New power plant output | MW																											661
Undergrounding project cost																$ 1,000,000,000
Entity share to complete project																500,000,000
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 had the effect of extending the DOJ review period until 30 days after each of Pepco Holdings and Exelon certified that it had substantially complied with the request.
Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage						8.80%	10.80%
Reserve for litigation filed																									$ 28,000,000	$ 28,000,000
ROE determined, description																									Each ten basis point reduction in the ROE would result in an increase in required reserves and a reduction of PHI’s operating income of $3.0 million.
Reduction in operating income																									$ 3,000,000
Expected [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate change																	$ 43,300,000							$ 68,400,000
Return on equity, percentage																	10.25%
Actual [Member]
Public Utilities, General Disclosures [Line Items]
Adjusted rate change																								66,200,000
District of Columbia [Member] | DC Undergrounding Task Force [Member]
Public Utilities, General Disclosures [Line Items]
Underground project cost																375,000,000
Maximum [Member]
Public Utilities, General Disclosures [Line Items]
New power plant output | MW																											700
Maximum [Member] | District of Columbia [Member] | Department of Transportation [Member]
Public Utilities, General Disclosures [Line Items]
Under grounding project costs covered by existing capital projects program																125,000,000
Minimum [Member]
Public Utilities, General Disclosures [Line Items]
New power plant output | MW																											650
Delmarva Power & Light Co/De [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate change										$ 39,000,000								$ 56,000,000		$ 42,000,000
Return on equity, percentage														9.70%						10.25%					10.80%
Approved rate change														$ 15,100,000
Return on equity, percentage														9.70%
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
Effect of proposed change on Gas Cost Rate								26.00%						7.40%
Basis-point																									0.50%	0.50%
New power plant output | MW																											661
Delmarva Power & Light Co/De [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage						8.80%	10.80%														8.70%				11.30%
Reserve for litigation filed																									$ 9,000,000	$ 9,000,000
ROE determined, description																									Each ten basis point reduction in the ROE would result in an increase in required reserves and a reduction of DPL's operating income of $1.0 million.
Reduction in operating income																									$ 1,000,000
Delmarva Power & Light Co/De [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
New power plant output | MW																											700
Delmarva Power & Light Co/De [Member] | Maximum [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage																					10.33%
Delmarva Power & Light Co/De [Member] | Minimum [Member]
Public Utilities, General Disclosures [Line Items]
New power plant output | MW																											650
Delmarva Power & Light Co/De [Member] | Minimum [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage																					6.78%
Atlantic City Electric Co [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate change					$ 33,900,000							$ 52,000,000
Return on equity, percentage																									10.80%
Approved rate change	$ 32,600,000		$ (1,300,000)	$ 33,900,000
Contributions received by ACE																									$ 11,000,000	$ 11,000,000
Consolidated tax adjustment calculation review period													5 years
Percentage of revenue requirement related to consolidated tax adjustment calculation													25.00%
Basis-point																									0.50%	0.50%
Atlantic City Electric Co [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage						8.80%	10.80%				8.70%														11.30%
Reserve for litigation filed																									$ 8,000,000	$ 8,000,000
ROE determined, description																									Each ten basis point reduction in the ROE would result in an increase in required reserves and a reduction of ACE's operating income of $0.8 million.
Reduction in operating income																									$ 800,000
Atlantic City Electric Co [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Contributions paid by ACE																										$ 1,000,000
Atlantic City Electric Co [Member] | Maximum [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage											10.33%
Atlantic City Electric Co [Member] | Minimum [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage											6.78%
Potomac Electric Power Co [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate change																								$ 68,400,000
Return on equity, percentage																			9.36%			10.25%		10.75%	10.80%
Approved rate change															$ 8,750,000				$ 27,900,000				$ 18,100,000
Return on equity, percentage															9.62%				9.36%				9.31%
Adjusted rate change									$ 37,400,000
Charges on cost of recovery																			$ 24,000,000
Basis-point																									0.50%	0.50%
New power plant output | MW																											661
Undergrounding project cost																1,000,000,000
Entity share to complete project																500,000,000
Potomac Electric Power Co [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage						8.80%	10.80%				8.70%										8.70%				11.30%
Reserve for litigation filed																									$ 11,000,000	$ 11,000,000
ROE determined, description																									Each ten basis point reduction in the ROE would result in an increase in required reserves and a reduction of Pepco's operating income of $1.2 million.
Reduction in operating income																									$ 1,200,000
Potomac Electric Power Co [Member] | Expected [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate change																	$ 43,300,000					$ 60,800,000
Return on equity, percentage																	10.25%
Potomac Electric Power Co [Member] | Actual [Member]
Public Utilities, General Disclosures [Line Items]
Adjusted rate change																								$ 66,200,000
Potomac Electric Power Co [Member] | District of Columbia [Member] | DC Undergrounding Task Force [Member]
Public Utilities, General Disclosures [Line Items]
Underground project cost																375,000,000
Potomac Electric Power Co [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
New power plant output | MW																											700
Potomac Electric Power Co [Member] | Maximum [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage											10.33%										10.33%
Potomac Electric Power Co [Member] | Maximum [Member] | District of Columbia [Member] | Department of Transportation [Member]
Public Utilities, General Disclosures [Line Items]
Under grounding project costs covered by existing capital projects program																$ 125,000,000
Potomac Electric Power Co [Member] | Minimum [Member]
Public Utilities, General Disclosures [Line Items]
New power plant output | MW																											650
Potomac Electric Power Co [Member] | Minimum [Member] | Federal Energy Regulatory Commission Return On Equity Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Return on equity, percentage											6.78%										6.78%