Revenue (Narrative) (Details)	12 Months Ended
Dec. 31, 2019
Cable Communications [Member] \| Residential [Member] \| Minimum [Member]
Revenue [Line Items]
Term of contract	1 year
Cable Communications [Member] \| Residential [Member] \| Maximum [Member]
Revenue [Line Items]
Term of contract	2 years
Cable Communications [Member] \| Business Services [Member]
Revenue [Line Items]
Annual revenue that estimates remaining performance obligations	6 months
Timing period of recognition	2 years
Cable Communications [Member] \| Business Services [Member] \| Minimum [Member]
Revenue [Line Items]
Term of contract	2 years
Cable Communications [Member] \| Business Services [Member] \| Maximum [Member]
Revenue [Line Items]
Term of contract	5 years
Cable Communications [Member] \| Advertising [Member] \| Minimum [Member]
Revenue [Line Items]
Expected payment terms	30 days
Cable Communications [Member] \| Advertising [Member] \| Maximum [Member]
Revenue [Line Items]
Expected payment terms	60 days
Cable Networks [Member] \| Distribution [Member]
Revenue [Line Items]
Expected payment terms	60 days
Cable Networks [Member] \| Advertising [Member]
Revenue [Line Items]
Expected payment terms	30 days
Broadcast Television [Member] \| Distribution [Member]
Revenue [Line Items]
Expected payment terms	60 days
Broadcast Television [Member] \| Advertising [Member]
Revenue [Line Items]
Expected payment terms	30 days
Filmed Entertainment [Member] \| Theatrical [Member]
Revenue [Line Items]
Expected payment terms	30 days
Filmed Entertainment [Member] \| Content Licensing [Member]
Revenue [Line Items]
Timing period of recognition	2 years
Filmed Entertainment [Member] \| Content Licensing [Member] \| Minimum [Member]
Revenue [Line Items]
Annual revenue that estimates remaining performance obligations	1 year
Filmed Entertainment [Member] \| Content Licensing [Member] \| Maximum [Member]
Revenue [Line Items]
Annual revenue that estimates remaining performance obligations	2 years
Filmed Entertainment [Member] \| Home Entertainment [Member] \| Minimum [Member]
Revenue [Line Items]
Expected payment terms	60 days
Filmed Entertainment [Member] \| Home Entertainment [Member] \| Maximum [Member]
Revenue [Line Items]
Expected payment terms	90 days
Sky Limited [Member] \| Direct To Consumer [Member]
Revenue [Line Items]
Term of contract	2 years
Timing period of recognition	2 years
NBCUniversal Media LLC [Member] \| Cable Networks [Member] \| Distribution [Member]
Revenue [Line Items]
Expected payment terms	60 days
NBCUniversal Media LLC [Member] \| Cable Networks [Member] \| Advertising [Member]
Revenue [Line Items]
Expected payment terms	30 days
NBCUniversal Media LLC [Member] \| Broadcast Television [Member] \| Distribution [Member]
Revenue [Line Items]
Expected payment terms	60 days
NBCUniversal Media LLC [Member] \| Broadcast Television [Member] \| Advertising [Member]
Revenue [Line Items]
Expected payment terms	30 days
NBCUniversal Media LLC [Member] \| Filmed Entertainment [Member] \| Theatrical [Member]
Revenue [Line Items]
Expected payment terms	30 days
NBCUniversal Media LLC [Member] \| Filmed Entertainment [Member] \| Content Licensing [Member]
Revenue [Line Items]
Timing period of recognition	2 years
NBCUniversal Media LLC [Member] \| Filmed Entertainment [Member] \| Content Licensing [Member] \| Minimum [Member]
Revenue [Line Items]
Annual revenue that estimates remaining performance obligations	1 year
NBCUniversal Media LLC [Member] \| Filmed Entertainment [Member] \| Content Licensing [Member] \| Maximum [Member]
Revenue [Line Items]
Annual revenue that estimates remaining performance obligations	2 years
NBCUniversal Media LLC [Member] \| Filmed Entertainment [Member] \| Home Entertainment [Member] \| Minimum [Member]
Revenue [Line Items]
Expected payment terms	60 days
NBCUniversal Media LLC [Member] \| Filmed Entertainment [Member] \| Home Entertainment [Member] \| Maximum [Member]
Revenue [Line Items]
Expected payment terms	90 days

Years of annual revenue that estimate revenue for performance obligations that have not been recognized as revenue

No definition available.

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Data Type:	xbrli:durationItemType
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Period Type:	duration

- Definition

Revenue, Performance Obligation, Description Of Payment Terms, Period

+ References

No definition available.

+ Details

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Namespace Prefix:	cmcsa_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue, Performance Obligation, Description Of Timing, Term Of Contract

+ References

No definition available.

+ Details

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Namespace Prefix:	cmcsa_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130556-203045

+ Details

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Balance Type:	na
Period Type:	instant

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration