Investments (Cost Method Investments) (Details) (Air Touch [Member], USD $) In Billions, unless otherwise specified	Mar. 31, 2013
Air Touch [Member]
Fair value of AirTouch preferred stock and the associated liability related to redeemable preferred shares	$ 1.8

The fair value of a cost method investment and the associated liability related to redeemable preferred shares.

No definition available.

Name:	cmcsa_FairValueOfCostMethodInvestmentAndAssociatedLiability
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant