Fair Value Disclosures - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	$ 25	$ 13
Financial instruments, assets	122	116
Financial instruments, liabilities	34	30
Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets	3
Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets		1
Derivative instruments, liabilities	4
Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	38	34
Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	35	35
Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	46	46
Executive deferred compensation plan liabilities	30	30
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	52	50
Financial instruments, liabilities	4
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets		1
Derivative instruments, liabilities	4
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	38	34
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	14	15
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	48	47
Financial instruments, liabilities	30	30
Significant Other Observable Inputs (Level 2) [Member] \| Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	21	20
Significant Other Observable Inputs (Level 2) [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	27	27
Executive deferred compensation plan liabilities	30	30
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	22	19
Significant Unobservable Inputs (Level 3) [Member] \| Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets	3
Significant Unobservable Inputs (Level 3) [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	19	19
Potomac Electric Power Co [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	5	3
Financial instruments, assets	80	77
Financial instruments, liabilities	7	7
Potomac Electric Power Co [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	5	3
Potomac Electric Power Co [Member] \| Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	34	33
Potomac Electric Power Co [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	41	41
Executive deferred compensation plan liabilities	7	7
Potomac Electric Power Co [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	18	16
Potomac Electric Power Co [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	5	3
Potomac Electric Power Co [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	13	13
Potomac Electric Power Co [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	44	43
Financial instruments, liabilities	7	7
Potomac Electric Power Co [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	21	20
Potomac Electric Power Co [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	23	23
Executive deferred compensation plan liabilities	7	7
Potomac Electric Power Co [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	18	18
Potomac Electric Power Co [Member] \| Significant Unobservable Inputs (Level 3) [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	18	18
Delmarva Power & Light Co/De [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	5
Financial instruments, assets	7	3
Financial instruments, liabilities	5	1
Delmarva Power & Light Co/De [Member] \| Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets		1
Derivative instruments, liabilities	4
Delmarva Power & Light Co/De [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	5
Delmarva Power & Light Co/De [Member] \| Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	1	1
Delmarva Power & Light Co/De [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	1	1
Executive deferred compensation plan liabilities	1	1
Delmarva Power & Light Co/De [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	6	2
Financial instruments, liabilities	4
Delmarva Power & Light Co/De [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets		1
Derivative instruments, liabilities	4
Delmarva Power & Light Co/De [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	5
Delmarva Power & Light Co/De [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	1	1
Delmarva Power & Light Co/De [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, liabilities	1	1
Delmarva Power & Light Co/De [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan liabilities	1	1
Delmarva Power & Light Co/De [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	1	1
Delmarva Power & Light Co/De [Member] \| Significant Unobservable Inputs (Level 3) [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	1	1
Atlantic City Electric Co [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	10	10
Atlantic City Electric Co [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	24	24
Atlantic City Electric Co [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Fund [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted cash equivalents	$ 24	$ 24

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- Definition

Financial instruments liabilities fair value disclosure.

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- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

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-Name Regulation S-X (SX)

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-Section 03

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-Subparagraph a, f, g

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Fair value portion of liability related to investment contracts for example, but not limited to, guaranteed investment contract or annuities.

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-Name Accounting Standards Codification

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The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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-Number 210

-Section 02

-Paragraph 1

-Article 5

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.5-02.1)

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