| Document and Entity Information | May 20, 2020 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | COMCAST CORP |
| Amendment Flag | false |
| Entity Central Index Key | 0001166691 |
| Document Type | 8-K |
| Document Period End Date | May 20, 2020 |
| Entity Incorporation State Country Code | PA |
| Entity File Number | 001-32871 |
| Entity Tax Identification Number | 27-0000798 |
| Entity Address, Address Line One | One Comcast Center |
| Entity Address, City or Town | Philadelphia |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 19103-2838 |
| City Area Code | (215) |
| Local Phone Number | 286-1700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Class B [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Class A Common Stock, $0.01 par value |
| Trading Symbol | CMCSA |
| Security Exchange Name | NASDAQ |
| Notes 0.250percent Due 2027 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 0.250% Notes due 2027 |
| Trading Symbol | CMCS27 |
| Security Exchange Name | NASDAQ |
| Notes 1.500percent Due 2029 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.500% Notes due 2029 |
| Trading Symbol | CMCS29 |
| Security Exchange Name | NASDAQ |
| Notes 0.750percentdue 2032 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 0.750% Notes due 2032 |
| Trading Symbol | CMCS32 |
| Security Exchange Name | NASDAQ |
| Notes 1.875percentdue 2036 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.875% Notes due 2036 |
| Trading Symbol | CMCS36 |
| Security Exchange Name | NASDAQ |
| Notes 1.250percentdue 2040 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.250% Notes due 2040 |
| Trading Symbol | CMCS40 |
| Security Exchange Name | NASDAQ |
| Guaranteed Notes 9.455 Percent Due 2022 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 9.455% Guaranteed Notes due 2022 |
| Trading Symbol | CMCSA/22 |
| Security Exchange Name | NYSE |
| Notes 5.50 Percent Due 2029 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.50% Notes due 2029 |
| Trading Symbol | CCGBP29 |
| Security Exchange Name | NYSE |
| Exchangeable Subordinated Debentures 2.0 Percent Due 2029 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 2.0% Exchangeable Subordinated Debentures due 2029 |
| Trading Symbol | CCZ |
| Security Exchange Name | NYSE |
