Investments (Cost Method Investments) (Details) - Air Touch [Member] $ in Billions	Mar. 31, 2016 USD ($)
Fair value of AirTouch preferred stock related to redeemable preferred shares	$ 1.7
Fair value of redeemable preferred shares	$ 1.7

The fair value of a cost method investment related to redeemable preferred shares.

No definition available.

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