Significant Transactions (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Significant Transactions [Line Items]
Transaction costs and transaction-related costs	$ 77	$ 198	$ 138
Write-off of assets related to merger agreement termination		$ 20

Amount of write-off of assets associated with the termination of the Time Warner Cable merger and the divestiture transactions recorded in depreciation and amortization expenses.

No definition available.

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