Fair Value Disclosures - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	$ 375	$ 122
Financial instruments, liabilities	29	34
Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets	18	3
Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, liabilities	1	4
Treasury Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and restricted cash equivalents	285
Restricted cash equivalents		38
Money Market Funds and Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	27	35
Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	45	46
Executive deferred compensation plan liabilities	28	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	298	52
Financial instruments, liabilities	1	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, liabilities	1	4
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and restricted cash equivalents	285
Restricted cash equivalents		38
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	14
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	40	48
Financial instruments, liabilities	28	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	14	21
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	26	27
Executive deferred compensation plan liabilities	28	30
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	37	22
Significant Unobservable Inputs (Level 3) [Member] \| Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets	18	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]
Financial instruments, assets	69	80
Financial instruments, liabilities	6	7
Potomac Electric Power Co [Member] \| Treasury Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and restricted cash equivalents	3
Restricted cash equivalents		5
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	26	34
Potomac Electric Power Co [Member] \| Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	40	41
Executive deferred compensation plan liabilities	6	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	15	18
Potomac Electric Power Co [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and restricted cash equivalents	3
Restricted cash equivalents		5
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	12	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	36	44
Financial instruments, liabilities	6	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	14	21
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	22	23
Executive deferred compensation plan liabilities	6	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]
Financial instruments, assets		7
Financial instruments, liabilities	2	5
Delmarva Power & Light Co/De [Member] \| Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, liabilities	1	4
Delmarva Power & Light Co/De [Member] \| Treasury Funds [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
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
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
Financial instruments, liabilities	1	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, liabilities	1	4
Delmarva Power & Light Co/De [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Funds [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
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
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
Atlantic City Electric Co [Member] \| Treasury Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and restricted cash equivalents	30	24
Atlantic City Electric Co [Member] \| Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] \| Treasury Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and restricted cash equivalents	$ 30	$ 24

Cash Equivalents And Restricted Cash Equivalents Fair Value Disclosure

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Financial instruments assets fair value disclosure.

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Financial instruments liabilities fair value disclosure.

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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.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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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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The aggregate value of all restricted investments.

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