Commitments and Contingencies (Split-Dollar Life Insurance Agreements) (Details) $ in Millions	1 Months Ended
Apr. 27, 2016 USD ($)
subsequent event member
Expense related to the settlement of a split-dollar life insurance liability	$ 116

Expense related to the settlement of a split-dollar life insurance liability recorded in other operating and administrative expenses

No definition available.

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