Fair Value Disclosures - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	$ 133	$ 122
Financial instruments, liabilities	30	34
Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets	3	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	48
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	35	35
Life Insurance Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Executive deferred compensation plan assets	47	46
Executive deferred compensation plan liabilities	29	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	63	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	48
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	15	14
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	47	48
Financial instruments, liabilities	29	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	20	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	27	27
Executive deferred compensation plan liabilities	29	30
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	23	22
Significant Unobservable Inputs (Level 3) [Member] \| Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments, assets	3	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	20	19
Potomac Electric Power Co [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	92	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	16
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	33	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	43	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	29	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	16
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	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	43	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	20	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	23	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	20	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	20	18
Delmarva Power & Light Co/De [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, assets	5	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]
Cash equivalents and restricted cash equivalents	4
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
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	5	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]
Cash equivalents and restricted cash equivalents	4
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
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	25	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	$ 25	$ 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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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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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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