v3.10.0.1
Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Jun. 30, 2018
Document And Entity Information [Line Items]    
Document Type 10-K  
Amendment Flag false  
Document Period End Date Dec. 31, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus FY  
Trading Symbol cmcsa  
Entity Registrant Name COMCAST CORP  
Entity Central Index Key 0001166691  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Emerging Growth Company false  
Entity Small Business false  
Entity Public Float   $ 149,068
Entity Current Reporting Status Yes  
Entity Well-known Seasoned Issuer Yes  
Entity Voluntary Filers No  
Entity Shell Company false  
NBCUniversal Media LLC [Member]    
Document And Entity Information [Line Items]    
Entity Registrant Name NBCUniversal Media, LLC  
Entity Central Index Key 0000902739  
Entity Filer Category Non-accelerated Filer  
Entity Emerging Growth Company false  
Entity Small Business false  
Entity Public Float   $ 0
Entity Current Reporting Status Yes  
Entity Well-known Seasoned Issuer Yes  
Entity Voluntary Filers No  
Entity Shell Company false  
Class A Common Stock [Member]    
Document And Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding 4,516,518,147  
Class B Common Stock [Member]    
Document And Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding 9,444,375