| Document and Entity Information | Feb. 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 | Feb. 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 A [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 |
| Exchangeable Subordinated Debentures 20 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 |
| Notes 550 Due 2029 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.50% Notes due 2029 |
| Trading Symbol | CCGBP29 |
| Security Exchange Name | NYSE |
| Guaranteed Notes 9455 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 |
