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Commitments and Contingencies (Rental expense) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Commitment And Contingencies [Line Items] |  |
Rental expense | $ 779 | $ 839
NBCUniversal Media LLC [Member] |  |
Commitment And Contingencies [Line Items] |  |
Rental expense | $ 286 | $ 274

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- Definition [Line Items] for Commitment And Contingencies [Table] + References No definition available. + Details
Name: | cmcsa_CommitmentAndContingenciesLineItems
Namespace Prefix: | cmcsa_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
Name: | us-gaap_LeaseAndRentalExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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