Organization - Additional Information (Detail) - USD ($)								12 Months Ended
	Jul. 24, 2015	Apr. 27, 2015	Jan. 26, 2015	Oct. 27, 2014	Jul. 29, 2014	Apr. 30, 2014	Apr. 29, 2014	Dec. 31, 2015	Dec. 31, 2014
Organization [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	$ 126,000,000
Termination fee							$ 259,000,000
Reimbursement of termination related expenses							$ 40,000,000
Potomac Electric Power Co [Member]
Organization [Line Items]
Common stock, par value								$ 0.01	$ 0.01
Delmarva Power & Light Co/De [Member]
Organization [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]
Organization [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
Terminations Due to New Acquisition Proposal [Member]
Organization [Line Items]
Termination fee							293,000,000
Terminations Due to New Acquisition Proposal [Member] | Delmarva Power & Light Co/De [Member]
Organization [Line Items]
Termination fee							293,000,000
Terminations Due to New Acquisition Proposal [Member] | Atlantic City Electric Co [Member]
Organization [Line Items]
Termination fee							293,000,000
Non-Voting Series A Preferred Stock [Member]
Organization [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]
Organization [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	$ 90,000,000	$ 90,000,000	$ 90,000,000	$ 90,000,000	$ 90,000,000
Non-voting Series A Preferred Stock, maximum aggregate consideration							180,000,000
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]
Organization [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	$ 90,000,000	$ 90,000,000	$ 90,000,000	$ 90,000,000	$ 90,000,000
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]
Organization [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	$ 90,000,000	$ 90,000,000	$ 90,000,000	$ 90,000,000	$ 90,000,000
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%
Maximum [Member] | Non-Voting Series A Preferred Stock [Member]
Organization [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]
Organization [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]
Organization [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]
Organization [Line Items]
Non-voting Series A Preferred Stock, maximum number of shares issued							18,000
Maximum [Member] | Exelon [Member]
Organization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							$ 40,000,000
Maximum [Member] | Exelon [Member] | Delmarva Power & Light Co/De [Member]
Organization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							40,000,000
Maximum [Member] | Exelon [Member] | Atlantic City Electric Co [Member]
Organization [Line Items]
Out-of-pocket expenses, connection with the Merger Agreement							$ 40,000,000