Recent Accounting Pronouncements (Narrative) (Details) - Share Based Compensation New Accounting Pronouncement [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2016
New Accounting Pronouncement, effect on income statement	$ 48	$ 159
New Accounting Pronouncement, effect on statement of cash flow		$ 411

Represents the amount that would have increased net cash provided by operating activities and decreased net cash provided by (used in) financing activities if the new accounting standard was adopted

No definition available.

Name:	cmcsa_NewAccountingPronouncementImpactOnStatementOfCashFlowIfAdopted
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration