Organization - Additional Information (Detail) - USD ($)								9 Months Ended
	Jul. 24, 2015	Apr. 27, 2015	Jan. 26, 2015	Oct. 27, 2014	Jul. 29, 2014	Apr. 30, 2014	Apr. 29, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Reorganization [Line Items]
Common stock, par value							$ 0.01	$ 0.01		$ 0.01
Right to receive cash under cancellation of shares							$ 27.25
Amount of Non Voting Series A Preferred Stock Purchase price								$ 54,000,000	$ 108,000,000
Termination fee							$ 259,000,000
Reimbursement of termination related expenses							$ 40,000,000
Potomac Electric Power Co [Member]
Reorganization [Line Items]
Common stock, par value							$ 0.01	$ 0.01		0.01
Right to receive cash under cancellation of shares							$ 27.25
Termination fee							$ 259,000,000
Reimbursement of termination related expenses							$ 40,000,000
Delmarva Power & Light Co/De [Member]
Reorganization [Line Items]
Common stock, par value							$ 0.01	2.25		2.25
Right to receive cash under cancellation of shares							$ 27.25
Termination fee							$ 259,000,000
Reimbursement of termination related expenses							$ 40,000,000
Atlantic City Electric Co [Member]
Reorganization [Line Items]
Common stock, par value							$ 0.01	3.00		$ 3.00
Right to receive cash under cancellation of shares							$ 27.25
Termination fee							$ 259,000,000
Reimbursement of termination related expenses							40,000,000
Non-Voting Series A Preferred Stock [Member]
Reorganization [Line Items]
Issuance of non-voting Series A Preferred Stock, shares issued						9,000
Non-voting Series A Preferred Stock, par value						$ 0.01
Non-voting Series A Preferred Stock, shares purchased price						$ 90,000,000
Non-voting Series A Preferred Stock, maximum number of shares issued	1,800	1,800	1,800	1,800	1,800
Amount of Non Voting Series A Preferred Stock Purchase price	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000
Non-voting Series A Preferred Stock, maximum aggregate consideration							180,000,000
Redemption of preferred stock at purchase price								10,000
Non-voting Series A Preferred Stock, cumulative, non-participating cash dividend						0.10%
Non-Voting Series A Preferred Stock [Member] | Potomac Electric Power Co [Member]
Reorganization [Line Items]
Issuance of non-voting Series A Preferred Stock, shares issued						9,000
Non-voting Series A Preferred Stock, par value						$ 0.01
Non-voting Series A Preferred Stock, shares purchased price						$ 90,000,000
Non-voting Series A Preferred Stock, maximum number of shares issued	1,800	1,800	1,800	1,800	1,800
Amount of Non Voting Series A Preferred Stock Purchase price	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000
Non-voting Series A Preferred Stock, maximum aggregate consideration							180,000,000
Redemption of preferred stock at purchase price								10,000
Non-voting Series A Preferred Stock, cumulative, non-participating cash dividend						0.10%
Non-Voting Series A Preferred Stock [Member] | Delmarva Power & Light Co/De [Member]
Reorganization [Line Items]
Issuance of non-voting Series A Preferred Stock, shares issued						9,000
Non-voting Series A Preferred Stock, par value						$ 0.01
Non-voting Series A Preferred Stock, shares purchased price						$ 90,000,000
Non-voting Series A Preferred Stock, maximum number of shares issued	1,800	1,800	1,800	1,800	1,800
Amount of Non Voting Series A Preferred Stock Purchase price	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000
Non-voting Series A Preferred Stock, maximum aggregate consideration							180,000,000
Redemption of preferred stock at purchase price								10,000
Non-voting Series A Preferred Stock, cumulative, non-participating cash dividend						0.10%
Non-Voting Series A Preferred Stock [Member] | Atlantic City Electric Co [Member]
Reorganization [Line Items]
Issuance of non-voting Series A Preferred Stock, shares issued						9,000
Non-voting Series A Preferred Stock, par value						$ 0.01
Non-voting Series A Preferred Stock, shares purchased price						$ 90,000,000
Non-voting Series A Preferred Stock, maximum number of shares issued	1,800	1,800	1,800	1,800	1,800
Amount of Non Voting Series A Preferred Stock Purchase price	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000	$ 18,000,000
Non-voting Series A Preferred Stock, maximum aggregate consideration							180,000,000
Redemption of preferred stock at purchase price								$ 10,000
Non-voting Series A Preferred Stock, cumulative, non-participating cash dividend						0.10%
Terminations Due to New Acquisition Proposal [Member]
Reorganization [Line Items]
Termination fee							293,000,000
Terminations Due to New Acquisition Proposal [Member] | Potomac Electric Power Co [Member]
Reorganization [Line Items]
Termination fee							293,000,000
Terminations Due to New Acquisition Proposal [Member] | Delmarva Power & Light Co/De [Member]
Reorganization [Line Items]
Termination fee							293,000,000
Terminations Due to New Acquisition Proposal [Member] | Atlantic City Electric Co [Member]
Reorganization [Line Items]
Termination fee							$ 293,000,000
Maximum [Member] | Non-Voting Series A Preferred Stock [Member]
Reorganization [Line Items]
Non-voting Series A Preferred Stock, maximum number of shares issued							18,000
Maximum [Member] | Non-Voting Series A Preferred Stock [Member] | Potomac Electric Power Co [Member]
Reorganization [Line Items]
Non-voting Series A Preferred Stock, maximum number of shares issued							18,000
Maximum [Member] | Non-Voting Series A Preferred Stock [Member] | Delmarva Power & Light Co/De [Member]
Reorganization [Line Items]
Non-voting Series A Preferred Stock, maximum number of shares issued							18,000
Maximum [Member] | Non-Voting Series A Preferred Stock [Member] | Atlantic City Electric Co [Member]
Reorganization [Line Items]
Non-voting Series A Preferred Stock, maximum number of shares issued							18,000
Maximum [Member] | Exelon [Member]
Reorganization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							$ 40,000,000
Maximum [Member] | Exelon [Member] | Potomac Electric Power Co [Member]
Reorganization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							40,000,000
Maximum [Member] | Exelon [Member] | Delmarva Power & Light Co/De [Member]
Reorganization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							40,000,000
Maximum [Member] | Exelon [Member] | Atlantic City Electric Co [Member]
Reorganization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							$ 40,000,000