Commitments and Contingencies (Narrative) (Details) $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($)
NBCUniversal Enterprise [Member]
Commitments And Contingencies [Line Items]
Cash payments for the redemption of redeemable subsidiary preferred stock	$ 725

Payments For Repurchase Of Redeemable Subsidiary Preferred Stock

No definition available.

Name:	cmcsa_PaymentsForRepurchaseOfRedeemableSubsidiaryPreferredStock
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration