v2.4.1.9

Investments (Fair Value Method Investments) (Details) (USD $) Share data in Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair value equity securities held as collateral | $ 523,000,000 cmcsa_EquitySecuritiesHeldAsCollateral | $ 3,959,000,000 cmcsa_EquitySecuritiesHeldAsCollateral |
Obligations under prepaid forward sale agreements | 119,000,000 cmcsa_ObligationsUnderPrepaidForwardSaleAgreements | 811,000,000 cmcsa_ObligationsUnderPrepaidForwardSaleAgreements |
Derivative component of prepaid forward sale agreements | 356,000,000 cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreements | 2,800,000,000 cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreements |
Total liabilities associated with prepaid forward sale agreements | 475,000,000 cmcsa_LiabilitiesRelatedToPrepaidForwardSaleAgreements | 3,611,000,000 cmcsa_LiabilitiesRelatedToPrepaidForwardSaleAgreements |
Fair Value Equity Securities Settlement | 3,200,000,000 cmcsa_FairValueEquitySecuritiesSettlement | 1,400,000,000 cmcsa_FairValueEquitySecuritiesSettlement |
Proceeds From Sales Of Businesses And Investments | 666,000,000 cmcsa_ProceedsFromSalesOfBusinessesAndInvestments | 1,083,000,000 cmcsa_ProceedsFromSalesOfBusinessesAndInvestments | 3,102,000,000 cmcsa_ProceedsFromSalesOfBusinessesAndInvestments
Clearwire LLC [Member] |  |  |
Proceeds from sale of available-for-sale securities |  | 443,000,000 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities / dei_LegalEntityAxis = cmcsa_ClearwireLlcMember |
Fair Value Investment Shares Sold |  | 89 cmcsa_FairValueInvestmentSharesSold / dei_LegalEntityAxis = cmcsa_ClearwireLlcMember |
Pre-tax gain realized on Clearwire |  | 443,000,000 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax / dei_LegalEntityAxis = cmcsa_ClearwireLlcMember |
Liberty Media Redemption Transaction [Member] |  |  |
Shares Redeemed |  | 6.3 cmcsa_SharesRedeemed / dei_LegalEntityAxis = cmcsa_LibertyMediaRedemptionTransactionMember |
Fair value of the shares delivered in the Liberty Media exchange transaction |  | 937,000,000 cmcsa_FairValueOfSharesDeliveredInTransaction / dei_LegalEntityAxis = cmcsa_LibertyMediaRedemptionTransactionMember |
Proceeds From Sales Of Businesses And Investments |  | $ 417,000,000 cmcsa_ProceedsFromSalesOfBusinessesAndInvestments / dei_LegalEntityAxis = cmcsa_LibertyMediaRedemptionTransactionMember |

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 Fair value method equity securities used to settle our obligations under prepaid forward sale agreements in the period. + References No definition available. + Details
Name: | cmcsa_FairValueEquitySecuritiesSettlement
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the amount of Clearwire shares sold to a third party. + References No definition available. + Details
Name: | cmcsa_FairValueInvestmentSharesSold
Namespace Prefix: | cmcsa_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the fair value of the shares delivered in the Liberty Media exchange transaction. + References No definition available. + Details
Name: | cmcsa_FairValueOfSharesDeliveredInTransaction
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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 cash inflow associated with the sale of businesses and investments during the period. + References No definition available. + Details
Name: | cmcsa_ProceedsFromSalesOfBusinessesAndInvestments
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the number of shares held by the Company as a fair value investment which were redeemed by the issuer. + References No definition available. + Details
Name: | cmcsa_SharesRedeemed
Namespace Prefix: | cmcsa_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount before tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e689-108580 + Details
Name: | us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26853-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=cmcsa_ClearwireLlcMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=cmcsa_LibertyMediaRedemptionTransactionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |