Segment Information (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment	6
Financial Data by Business Segment
Revenue	$ 26,859	$ 22,791
Adjusted EBITDA	8,553	7,244
Depreciation and Amortization	3,320	2,599
Capital Expenditures	2,092	1,973
Cash Paid for Intangible Assets	$ 547	419
NBCUniversal Media LLC [Member]
Segment Reporting Information [Line Items]
Number of reportable segments \| segment	4
Financial Data by Business Segment
Revenue	$ 8,345	9,530
Adjusted EBITDA	2,347	2,285
Depreciation and Amortization	515	510
Capital Expenditures	453	269
Cash Paid for Intangible Assets	71	130
Cable Communications [Member]
Financial Data by Business Segment
Revenue	14,280	13,703
Cable Networks [Member]
Financial Data by Business Segment
Revenue	2,868	3,157
Cable Networks [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	2,868	3,157
Broadcast Television [Member]
Financial Data by Business Segment
Revenue	2,467	3,497
Broadcast Television [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	2,467	3,497
Filmed Entertainment [Member]
Financial Data by Business Segment
Revenue	1,768	1,647
Filmed Entertainment [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	1,768	1,647
Theme Parks [Member]
Financial Data by Business Segment
Revenue	1,276	1,281
Theme Parks [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	1,276	1,281
NBCUniversal Segments [Member]
Financial Data by Business Segment
Revenue	8,313	9,497
Adjusted EBITDA	2,337	2,271
Depreciation and Amortization	515	510
Capital Expenditures	453	269
Cash Paid for Intangible Assets	71	130
Sky Limited [Member]
Financial Data by Business Segment
Revenue	4,797	0
Operating Segments [Member] \| Cable Communications [Member]
Financial Data by Business Segment
Revenue	14,280	13,703
Adjusted EBITDA	5,728	5,217
Depreciation and Amortization	2,035	2,061
Capital Expenditures	1,363	1,691
Cash Paid for Intangible Assets	323	287
Operating Segments [Member] \| Cable Networks [Member]
Financial Data by Business Segment
Revenue	2,868	3,157
Adjusted EBITDA	1,262	1,254
Depreciation and Amortization	182	189
Capital Expenditures	6	3
Cash Paid for Intangible Assets	2	4
Operating Segments [Member] \| Cable Networks [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	2,868	3,157
Adjusted EBITDA	1,262	1,254
Depreciation and Amortization	182	189
Capital Expenditures	6	3
Cash Paid for Intangible Assets	2	4
Operating Segments [Member] \| Broadcast Television [Member]
Financial Data by Business Segment
Revenue	2,467	3,497
Adjusted EBITDA	387	507
Depreciation and Amortization	39	34
Capital Expenditures	13	30
Cash Paid for Intangible Assets	3	72
Operating Segments [Member] \| Broadcast Television [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	2,467	3,497
Adjusted EBITDA	387	507
Depreciation and Amortization	39	34
Capital Expenditures	13	30
Cash Paid for Intangible Assets	3	72
Operating Segments [Member] \| Filmed Entertainment [Member]
Financial Data by Business Segment
Revenue	1,768	1,647
Adjusted EBITDA	364	203
Depreciation and Amortization	19	28
Capital Expenditures	4	7
Cash Paid for Intangible Assets	5	6
Operating Segments [Member] \| Filmed Entertainment [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	1,768	1,647
Adjusted EBITDA	364	203
Depreciation and Amortization	19	28
Capital Expenditures	4	7
Cash Paid for Intangible Assets	5	6
Operating Segments [Member] \| Theme Parks [Member]
Financial Data by Business Segment
Revenue	1,276	1,281
Adjusted EBITDA	498	495
Depreciation and Amortization	162	155
Capital Expenditures	394	182
Cash Paid for Intangible Assets	19	16
Operating Segments [Member] \| Theme Parks [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	1,276	1,281
Adjusted EBITDA	498	495
Depreciation and Amortization	162	155
Capital Expenditures	394	182
Cash Paid for Intangible Assets	19	16
Operating Segments [Member] \| Sky Limited [Member]
Financial Data by Business Segment
Revenue	4,797
Adjusted EBITDA	663
Depreciation and Amortization	741
Capital Expenditures	259
Cash Paid for Intangible Assets	151
Corporate, Non-Segment [Member]
Financial Data by Business Segment
Revenue	108	243
Adjusted EBITDA	(187)	(185)
Depreciation and Amortization	29	28
Capital Expenditures	17	13
Cash Paid for Intangible Assets	2	2
Corporate, Non-Segment [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	51	51
Adjusted EBITDA	(164)	(174)
Depreciation and Amortization	113	104
Capital Expenditures	36	47
Cash Paid for Intangible Assets	42	32
Corporate, Non-Segment [Member] \| NBCUniversal Segments [Member]
Financial Data by Business Segment
Revenue	17	14
Adjusted EBITDA	(174)	(188)
Depreciation and Amortization	113	104
Capital Expenditures	36	47
Cash Paid for Intangible Assets	42	32
Eliminations [Member]
Financial Data by Business Segment
Revenue	(639)	(652)
Adjusted EBITDA	12	(59)
Depreciation and Amortization	0	0
Capital Expenditures	0	0
Cash Paid for Intangible Assets	0	0
Eliminations [Member] \| NBCUniversal Media LLC [Member]
Financial Data by Business Segment
Revenue	(85)	(103)
Adjusted EBITDA	0	0
Depreciation and Amortization	0	0
Capital Expenditures	0	0
Cash Paid for Intangible Assets	0	0
Eliminations [Member] \| NBCUniversal Segments [Member]
Financial Data by Business Segment
Revenue	(83)	(99)
Adjusted EBITDA	0	0
Depreciation and Amortization	0	0
Capital Expenditures	0	0
Cash Paid for Intangible Assets	$ 0	$ 0

The measure used to evaluate the profitability of operating segments.

No definition available.

Name:	cmcsa_AdjustedEBITDA
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_DepreciationAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow to acquire asset without physical form usually arising from contractual or other legal rights, excluding goodwill.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquireIntangibleAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

+ References

+ Details

Name:	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

- References

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cmcsa_CableNetworksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cmcsa_BroadcastTelevisionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cmcsa_FilmedEntertainmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cmcsa_ThemeParksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cmcsa_NBCUniversalSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_MaterialReconcilingItemsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: