v2.4.0.8

Investments (Fair Value Method Investments) (Details) (USD $) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2014 | Dec. 31, 2013
Fair value equity securities held as collateral | $ 2,250,000,000 | $ 3,959,000,000
Obligations under prepaid forward sale agreements | 470,000,000 | 811,000,000
Derivative component of prepaid forward sale agreements | 1,586,000,000 | 2,800,000,000
Total liabilities associated with prepaid forward sale agreements | 2,056,000,000 | 3,611,000,000
Prepaid forward sale obligations fair value | 2,100,000,000 |
Equity securities held as collateral under our prepaid forward sale agreements acquired during the period | $ 1,500,000,000 |

X
- Definition This element represents the fair value as of the balance sheet date of embedded derivative liabilities as a component of prepaid forward sale agreements valued on a recurring basis. + References No definition available. + Details
Name: | cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreements
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair value method equity securities held as collateral related to our obligations under prepaid forward sale agreements. + References No definition available. + Details
Name: | cmcsa_EquitySecuritiesHeldAsCollateral
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The cash outflow associated with the purchase of all investments during the period that were used to serve as collateral for obligations under prepaid forward sale agreements. + References No definition available. + Details
Name: | cmcsa_EquitySecuritiesPurchasedInPeriod
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The carrying value of the liabilities related to prepaid forward sale agreements. + References No definition available. + Details
Name: | cmcsa_LiabilitiesRelatedToPrepaidForwardSaleAgreements
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The carrying value of obligations related to prepaid forward sale agreements. + References No definition available. + Details
Name: | cmcsa_ObligationsUnderPrepaidForwardSaleAgreements
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The estimated fair value of our prepaid forward sale obligations. + References No definition available. + Details
Name: | cmcsa_PrepaidForwardSaleObligationsFairValue
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant