v2.4.1.9
Significant Transactions (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2014
Mar. 19, 2013
Significant Transactions [Line Items]      
Purchase of NBCUniversal noncontrolling interest $ 16,700,000,000cmcsa_ConsiderationTransferredToAcquireAdditionalInterestInSubsidiaries    
Consideration transferred related to the Redemption Transaction and purchase of certain properties 725,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource    
Remaining equity interest acquired     49.00%cmcsa_NoncontrollingInterestAcquired
Company Shares Exchanged For Each Acquiree Share   2.875cmcsa_CompanySharesExchangedForEachAcquireeShare  
NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Purchase of NBCUniversal noncontrolling interest 16,700,000,000cmcsa_ConsiderationTransferredToAcquireAdditionalInterestInSubsidiaries
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Consideration transferred related to the Redemption Transaction and purchase of certain properties 725,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Remaining equity interest acquired     49.00%cmcsa_NoncontrollingInterestAcquired
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
Redemption percentage 15.00%cmcsa_RedemptionPercentage
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Comcast Corporation [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Ownership percentage in NBCUniversal Holdings 96.00%cmcsa_OwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_ComcastCorporationMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Comcast Cash On Hand      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 11,400,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_CashMember
   
Comcast Cash On Hand | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 11,400,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_CashMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
NBCUniversal Cash On Hand [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Portion Of Consideration Transferred Funded By NBCUniversal 4,600,000,000cmcsa_PortionOfConsiderationTransferredFundedByNbcuniversal
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalCashOnHandMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
NBCUniversal Enterprise Senior Unsecured Debt Securities [Member]      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 4,000,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_SeniorNotesMember
   
NBCUniversal Enterprise Senior Unsecured Debt Securities [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 4,000,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_SeniorNotesMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Commercial Paper Program      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 750,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_CommercialPaperMember
   
Commercial Paper Program | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 750,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_CommercialPaperMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Nbcuniversal Holdings Preferred Stock [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Preferred stock dividend rate percentage 8.25%us-gaap_PreferredStockDividendRatePercentage
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalHoldingsPreferredStockMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Preferred stock liquidition preference value 9,400,000,000us-gaap_PreferredStockLiquidationPreferenceValue
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalHoldingsPreferredStockMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Preferred stock dividend reset rate percentage description On March 1, 2018, and thereafter on every fifth anniversary of such date, this rate will reset to 7.44% plus the yield on actively traded United States Treasury securities having a 5 year maturity.    
Thirty Rockefeller Plaza And Englewood Cliffs [Member]      
Significant Transactions [Line Items]      
Payments related to acquisition of real estate properties 1,400,000,000cmcsa_PaymentsRelatedToAcquisitionOfBuildings
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_ThirtyRockefellerPlazaAndEnglewoodCliffsMember
   
Thirty Rockefeller Plaza And Englewood Cliffs [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Payments related to acquisition of real estate properties 1,400,000,000cmcsa_PaymentsRelatedToAcquisitionOfBuildings
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_ThirtyRockefellerPlazaAndEnglewoodCliffsMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
NBCUniversal Enterprise [Member]      
Significant Transactions [Line Items]      
Preferred stock dividend rate percentage 5.25%us-gaap_PreferredStockDividendRatePercentage
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalEnterpriseMember
   
Fair value of redeemable preferred stock 741,000,000cmcsa_FairValueAmountOfRedeemablePreferredStock
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalEnterpriseMember
751,000,000cmcsa_FairValueAmountOfRedeemablePreferredStock
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalEnterpriseMember
 
NBCUniversal Enterprise [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Ownership percentage in NBCUniversal Holdings 4.00%cmcsa_OwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalEnterpriseMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember
   
Merger Agreement Undertaking Number Of Divested Customers [Member]      
Significant Transactions [Line Items]      
Number of customers   3,000,000cmcsa_NumberOfCustomers
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_MergerAgreementUndertakingNumberOfDivestedCustomersMember
 
Comcast Corporate Shareholders [Member]      
Significant Transactions [Line Items]      
Ownership Percentage In SpinCo   67.00%cmcsa_OwnershipPercentageInSpinco
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_ComcastCorporateShareholdersMember
 
New Charter [Member]      
Significant Transactions [Line Items]      
Ownership Percentage In SpinCo   33.00%cmcsa_OwnershipPercentageInSpinco
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NewCharterMember
 
Transaction-Related Expenses [Member]      
Significant Transactions [Line Items]      
Other operating and administrative expenses   237,000,000cmcsa_Transactioncostsandtransactionrelatedcostsinoperatingcostsandexpenses
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_TransactionRelatedExpensesMember
 
NBCUniversal Enterprise Credit Facility [Member]      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties 1,250,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalEnterpriseCreditFacilityMember
   
NBCUniversal Enterprise Credit Facility [Member] | NBCUniversal Media, LLC [Member]      
Significant Transactions [Line Items]      
Consideration transferred related to the Redemption Transaction and purchase of certain properties $ 1,250,000,000cmcsa_ConsiderationTransferredRelatedToRedemptionTransactionByFundingSource
/ us-gaap_BusinessAcquisitionAxis
= cmcsa_NbcuniversalEnterpriseCreditFacilityMember
/ dei_LegalEntityAxis
= cmcsa_NBCUniversalMediaLLCMember