Income Taxes - Reconciliation of Beginning and Ending Balances of Unrecognized Tax Benefits (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	$ 831us-gaap_UnrecognizedTaxBenefits	$ 200us-gaap_UnrecognizedTaxBenefits	$ 357us-gaap_UnrecognizedTaxBenefits
Tax positions related to current year, Additions	4	3	1
Tax positions related to current year, Reductions	(2)
Tax positions related to prior years, Additions	27	646	79
Tax positions related to prior years, Reductions	(10)	(12)	(235)
Settlements		(6)	(2)
Unrecognized Tax Benefits, Ending balance	850us-gaap_UnrecognizedTaxBenefits	831us-gaap_UnrecognizedTaxBenefits	200us-gaap_UnrecognizedTaxBenefits
Potomac Electric Power Co [Member]
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	101	91	173
Tax positions related to current year, Additions	1	1
Tax positions related to current year, Reductions	(2)
Tax positions related to prior years, Additions	1	12	60
Tax positions related to prior years, Reductions	(4)	(3)	(142)
Unrecognized Tax Benefits, Ending balance	97	101	91
Delmarva Power & Light Co/De [Member]
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	9	9	35
Tax positions related to current year, Additions	1
Tax positions related to prior years, Additions	13
Tax positions related to prior years, Reductions	(1)		(26)
Unrecognized Tax Benefits, Ending balance	22	9	9
Atlantic City Electric Co [Member]
Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits, Beginning balance	9	17	79
Tax positions related to current year, Additions	1	2	1
Tax positions related to prior years, Additions	5	1	8
Tax positions related to prior years, Reductions	(2)	(5)	(69)
Settlements		(6)	(2)
Unrecognized Tax Benefits, Ending balance	$ 13	$ 9	$ 17

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Unrecognized Tax Benefit

-URI http://asc.fasb.org/extlink&oid=6527854

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15A

-Subparagraph (a)

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of increase in unrecognized tax benefits resulting from settlements with taxing authorities.

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