Stock-Based Compensation Plans - Narrative (Details) shares in Millions	12 Months Ended
Dec. 31, 2019 shares
Performance Shares [Member]
Common Stock Narrative Information [Line Items]
Percentage to be settled as common stock	50.00%
Percentage to be settled as cash	50.00%
Percentage to be settled as cash if ownership requirements are met	100.00%
Minimum [Member] \| Restricted Stock Units (RSUs) [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	3 years
Maximum [Member] \| Restricted Stock Units (RSUs) [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	5 years
LTIP [Member]
Common Stock Narrative Information [Line Items]
Shares authorized	12

Common Stock Narrative Information [Line Items]

No definition available.

Name:	exc_CommonStockNarrativeInformationLineItems
Namespace Prefix:	exc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation by Share-based Payment Award, Percentage of Awards to be Settled in Cash

+ References

No definition available.

+ Details

Name:	exc_SharebasedCompensationbySharebasedPaymentAwardPercentageofAwardstobeSettledinCash
Namespace Prefix:	exc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation by Share-based Payment Award, Percentage of Awards to be Settled in Cash if Agreement Conditions are Met

+ References

No definition available.

+ Details

Name:	exc_SharebasedCompensationbySharebasedPaymentAwardPercentageofAwardstobeSettledinCashifAgreementConditionsareMet
Namespace Prefix:	exc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation by Share-based Payment Award, Percentage of Awards to be Settled in Stock

+ References

No definition available.

+ Details

Name:	exc_SharebasedCompensationbySharebasedPaymentAwardPercentageofAwardstobeSettledinStock
Namespace Prefix:	exc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=exc_LongTermIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: