Investments (Fair Value Method Investments) (Details) $ in Millions	9 Months Ended
Sep. 30, 2015 USD ($)
Prepaid forward sale liabilities carrying value	$ 0
Fair Value Equity Securities Settlement	$ 517

Fair value method equity securities used to settle our obligations under prepaid forward sale agreements in the period.

No definition available.

Name:	cmcsa_FairValueEquitySecuritiesSettlement
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration