The cash outflow associated with a capital contribution to a VIE accounted for as an equity method investment

No definition available.

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

- Definition

Carrying amount of obligation presented separately as collateralized obligation that is secured by assets.

+ References

No definition available.

+ Details

Name:	cmcsa_CollateralizedObligation
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of obligation presented separately as collateralized obligation that is secured by assets.

+ References

No definition available.

+ Details

Name:	cmcsa_CollateralizedObligationFairValue
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from amounts received from the issuance of an obligation that is secured by assets and is presented separately as proceeds from collateralized obligation.

+ References

No definition available.

+ Details

Name:	cmcsa_ProceedsFromCollateralizedObligation
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=cmcsa_AtairosMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=cmcsa_HuluMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments (Equity Method Investments) (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Equity method investment	$ 6,921			$ 6,006
Collateralized obligation	5,169			5,168
Atairos [Member]
Schedule of Equity Method Investments [Line Items]
Cash capital contributions for equity method investments	24	$ 172
Equity method investment	4,800			3,900
Hulu [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from collateralized obligation			$ 5,200
Collateralized obligation	5,200			5,200
Fair value of collateralized obligation	$ 5,200			$ 5,200