| Document and Entity Information Document | Aug. 12, 2021 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Aug. 12, 2021 |
| Entity File Number | 001-01839 |
| Entity Registrant Name | COMMONWEALTH EDISON COMPANY |
| Entity Tax Identification Number | 36-0938600 |
| Entity Incorporation, State or Country Code | IL |
| Entity Address, Address Line One | 440 South LaSalle Street |
| Entity Address, City or Town | Chicago |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60605-1028 |
| City Area Code | (312) |
| Local Phone Number | 394-4321 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Central Index Key | 0000022606 |
| Amendment Flag | false |
| Entity Emerging Growth Company | false |
