Commitments and Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Feb. 12, 2013
Collateral liability for unconsolidated subsidiary debt		$ 482
Cash paid for settlement of collateral liability	602
Station Venture Senior Secured Note Due In 2023 [Member]
Debt of unconsolidated variable interest entity	$ 816
Maturity year	2023

The cash paid during the period for the settlement of a collateral liability held on the balance sheet related to a senior secured note held by a nonconsolidated subsidiary.

No definition available.

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