Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Jun. 30, 2017
Document And Entity Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
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 Public Float		$ 181,936
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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 Public Float		$ 0
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Class A Common Stock [Member]
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding	4,635,063,642
Class B Common Stock [Member]
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding	9,444,375