Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Derivatives Fair Value Footnotes [Abstract]
Cash collateral excluded	185,000,000
Fair Value, Inputs, Level 3 [Member]
Derivatives Fair Value Footnotes [Abstract]
Cash collateral excluded		172,000,000
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	892,000,000	893,000,000
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Discounted Cash Flow [Member] \| Minimum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	17	15
Forward gas price assets	1.68	1.52
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Discounted Cash Flow [Member] \| Maximum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	121	120
Forward gas price assets	13.69	14.02
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Option Model Valuation Technique [Member] \| Minimum [Member]
Fair Value Inputs [Abstract]
Volatility percentage	8.00%	8.00%
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Option Model Valuation Technique [Member] \| Maximum [Member]
Fair Value Inputs [Abstract]
Volatility percentage	172.00%	257.00%
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Proprietary Trading [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	11,000,000	15,000,000
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Proprietary Trading [Member] \| Discounted Cash Flow [Member] \| Minimum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	17	15
Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Proprietary Trading [Member] \| Discounted Cash Flow [Member] \| Maximum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	95	117
Commonwealth Edison Co [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	241,000,000	207,000,000
Commonwealth Edison Co [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Discounted Cash Flow [Member] \| Minimum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	3.50%	3.50%
Forward heat rate	(8.00%)	(8.00%)
Renewable factor	86.00%	86.00%
Commonwealth Edison Co [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Discounted Cash Flow [Member] \| Maximum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	8.00%	8.00%
Forward heat rate	(9.00%)	(9.00%)
Renewable factor	126.00%	126.00%
All Regions excluding New England [Member] \| Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member] \| Discounted Cash Flow [Member] \| Maximum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	107	97
Forward gas price assets	8.19	8.14
All Regions excluding New England [Member] \| Exelon Generation Co L L C [Member] \| Fair Value, Inputs, Level 3 [Member] \| Proprietary Trading [Member] \| Discounted Cash Flow [Member] \| Maximum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	55	76

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This element represents the Level 3 cash collateral value excluded from the significant inputs to the forward curve used to value swap contracts

+ References

No definition available.

+ Details

Name:	exc_FairValueInputsCashCollateralExcluded
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- Definition

This element represents the forward power market price used to calculate the change in fair value of the derivative instrument.

+ References

No definition available.

+ Details

Name:	exc_FairValueInputsForwardGasPriceAssets
Namespace Prefix:	exc_
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- Definition

This element represents the factor that is multiplied with the forward price of natural gas to calculate a forward power curve. Natural gas has greater liquidity tenor than power and is commonly used to calculate this curve.

+ References

No definition available.

+ Details

Name:	exc_FairValueInputsForwardHeatRate
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Period Type:	duration

- Definition

This element represents the forward power market price used to calculate the change in fair value of the derivative instrument.

+ References

No definition available.

+ Details

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- Definition

This element represents a factor to adjust an around the clock forward power price and quantity to be in line with an applicable renewable facility. Solar units generally produce more power in the summer days and wind units generally produce more power in the spring and fall evenings.

+ References

No definition available.

+ Details

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- Definition

Fair value, after offset of derivative liability, of 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (c)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section 55

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

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-Paragraph 8

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-URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41678-113959

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- Definition

Fair value, after offset of derivative asset, of 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

+ Details

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

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