Recent Accounting Pronouncements (Narrative) (Details) - Share Based Compensation New Accounting Pronouncement [Member] $ in Millions	3 Months Ended
Mar. 31, 2016 USD ($)
New Accounting Pronouncement, effect on income statement	$ 111
New Accounting Pronouncement, effect on statement of cash flow	$ 289

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