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v3.19.3.a.u2

Segment Information (Narrative) (Details) customer in Millions, activated_line in Millions | Dec. 31, 2019 activated_line customer
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Cable Communications [Member] |
Segment Reporting Information [Line Items] |
Total customer relationships | 31.5
Cable Communications [Member] | High-speed Internet Customers [Member] |
Segment Reporting Information [Line Items] |
Number of customers | 28.6
Cable Communications [Member] | Video Customers [Member] |
Segment Reporting Information [Line Items] |
Number of customers | 21.3
Cable Communications [Member] | Voice Customers [Member] |
Segment Reporting Information [Line Items] |
Number of customers | 11.3
Cable Communications [Member] | Security and Automation Customers [Member] |
Segment Reporting Information [Line Items] |
Number of customers | 1.4
Cable Communications [Member] | Wireless Phone Service [Member] |
Segment Reporting Information [Line Items] |
Number of activated lines | activated_line | 2.1
Cable Communications [Member] | Residential [Member] |
Segment Reporting Information [Line Items] |
Total customer relationships | 29.1
Cable Communications [Member] | Business Customers [Member] |
Segment Reporting Information [Line Items] |
Total customer relationships | 2.4
Sky Limited [Member] |
Segment Reporting Information [Line Items] |
Retail customer relationships | 24.0

X
- Definition Number Of Activated Lines + References No definition available. + Details
Name: | cmcsa_NumberOfActivatedLines
Namespace Prefix: | cmcsa_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of customers that subscribe to our services. + References No definition available. + Details
Name: | cmcsa_NumberOfCustomers
Namespace Prefix: | cmcsa_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of residential and business customers that subscribe to at least one of our four primary services of video, high-speed Internet, voice, and security and automation. + References No definition available. + Details
Name: | cmcsa_TotalCustomerRelationships
Namespace Prefix: | cmcsa_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of retail customers that subscribe to at least one of the primary services of video, high-speed internet, voice, and wireless phone services. + References No definition available. + Details
Name: | cmcsa_TotalRetailCustomerRelationships
Namespace Prefix: | cmcsa_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=cmcsa_CableCommunicationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=cmcsa_HighSpeedInternetCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=cmcsa_VideoCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=cmcsa_VoiceCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=cmcsa_SecurityAndAutomationCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=cmcsa_WirelessLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_MajorCustomersAxis=cmcsa_ResidentialCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_MajorCustomersAxis=cmcsa_BusinessCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=cmcsa_SkyLimitedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |