Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Mark-to-market derivative assets, current	$ 573	$ 679
Derivative Asset, Noncurrent	466	508
Mark-to-market derivative liabilities, current	(204)	(247)
Mark-to-market derivative liabilities, noncurrent	(440)	(393)
Margin Deposit Assets	383	511
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	570	675
Derivative Asset, Noncurrent	466	508
Total mark-to-market derivative assets	1,036	1,183
Mark-to-market derivative liabilities, current	(190)	(236)
Mark-to-market derivative liabilities, noncurrent	(421)	(380)
Total mark-to-market derivative liabilities	(611)	(616)
Total mark-to-market derivative net assets (liabilities)	425	567
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	570	675
Derivative Asset, Noncurrent	466	508
Mark-to-market derivative liabilities, current	(173)	(215)
Mark-to-market derivative liabilities, noncurrent	(154)	(123)
Margin Deposit Assets	383	511
Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	570	675
Derivative Asset, Noncurrent	466	508
Total mark-to-market derivative assets	1,036	1,183
Mark-to-market derivative liabilities, current	(159)	(204)
Mark-to-market derivative liabilities, noncurrent	(134)	(111)
Total mark-to-market derivative liabilities	(293)	(315)
Total mark-to-market derivative net assets (liabilities)	743	868
Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities, current	(31)	(32)
Mark-to-market derivative liabilities, noncurrent	(287)	(269)
Collateral [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	179	287
Derivative Asset, Noncurrent	77	122
Total mark-to-market derivative assets	256	409
Mark-to-market derivative liabilities, current	211	357
Mark-to-market derivative liabilities, noncurrent	109	163
Total mark-to-market derivative liabilities	320	520
Total mark-to-market derivative net assets (liabilities)	576	929
Netting [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	(2,919)	(3,190)
Derivative Asset, Noncurrent	(1,213)	(877)
Total mark-to-market derivative assets	(4,132)	(4,067)
Mark-to-market derivative liabilities, current	2,919	3,190
Mark-to-market derivative liabilities, noncurrent	1,213	877
Total mark-to-market derivative liabilities	4,132	4,067
Total mark-to-market derivative net assets (liabilities)	0	0
Not Designated as Hedging Instrument, Economic Hedge [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	3,240	3,506
Derivative Asset, Noncurrent	1,582	1,238
Total mark-to-market derivative assets	4,822	4,744
Mark-to-market derivative liabilities, current	(3,243)	(3,713)
Mark-to-market derivative liabilities, noncurrent	(1,442)	(1,140)
Total mark-to-market derivative liabilities	(4,685)	(4,853)
Total mark-to-market derivative net assets (liabilities)	137	(109)
Not Designated as Hedging Instrument, Economic Hedge [Member] \| Commonwealth Edison Co [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	0	0
Derivative Asset, Noncurrent	0	0
Total mark-to-market derivative assets	0	0
Mark-to-market derivative liabilities, current	(31)	(32)
Mark-to-market derivative liabilities, noncurrent	(287)	(269)
Total mark-to-market derivative liabilities	(318)	(301)
Total mark-to-market derivative net assets (liabilities)	(318)	(301)
Not Designated as Hedging Instrument [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	70	72
Derivative Asset, Noncurrent	20	25
Total mark-to-market derivative assets	90	97
Mark-to-market derivative liabilities, current	(46)	(38)
Mark-to-market derivative liabilities, noncurrent	(14)	(11)
Total mark-to-market derivative liabilities	(60)	(49)
Total mark-to-market derivative net assets (liabilities)	$ 30	$ 48

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeAssetsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilitiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_MarginDepositAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CommodityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=exc_ExelonGenerationCoLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=exc_CommonwealthEdisonCoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=exc_CollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=exc_NettingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NotDesignatedAsHedgingInstrumentEconomicHedgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NotDesignatedAsHedgingInstrumentTradingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: