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

Deferred Gain Recognized On Dilution Of Equity Method Investment

+ References

No definition available.

+ Details

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

- Definition

Fair market value floor representing minimum total equity value of investment for put and call option.

+ References

No definition available.

+ Details

Name:	cmcsa_EquityMethodInvestmentFairMarketValueFloorForPutAndCallOption
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Equity Method Investment, Maximum Total Capital Call In Annual Period

+ References

No definition available.

+ Details

Name:	cmcsa_EquityMethodInvestmentMaximumTotalCapitalCallInAnnualPeriod
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Equity Method Investment, Minimum Guaranteed Proceeds Floor

+ References

No definition available.

+ Details

Name:	cmcsa_EquityMethodInvestmentMinimumGuaranteedProceedsFloorForPutAndCallOption
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Equity Method Investment, Ownership Percentage, Floor

+ References

No definition available.

+ Details

Name:	cmcsa_EquityMethodInvestmentOwnershipPercentageFloor
Namespace Prefix:	cmcsa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Equity Method Investment, Ownership Percentage Representing Fair Market Value Floor For Put And Call Option

+ References

No definition available.

+ Details

Name:	cmcsa_EquityMethodInvestmentOwnershipPercentageRepresentingFairMarketValueFloorForPutAndCallOption
Namespace Prefix:	cmcsa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount of investment presented separately as investment securing collateralized obligation that is partially securing an obligation.

+ References

No definition available.

+ Details

Name:	cmcsa_InvestmentSecuringCollateralizedObligation
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of gain (loss) on sale or disposal of an equity method investment.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(7)(c))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(9)(a))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

+ Details

Name:	us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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=115929826&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=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of gain (loss) from sale and disposal of integrated set of activities and assets capable of being conducted and managed for purpose of providing return in form of dividend, lower cost, or other economic benefit to investor, owner, member and participant.

+ References

+ Details

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

- Definition

Amount of income (loss) for proportionate share of equity method investee's income (loss).

+ References

Reference 1: 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(12))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(10))
-URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263

Reference 3: 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=118932676&loc=d3e3602-108585

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(f))
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 6: 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

+ Details

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

- Definition

The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ 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 (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585

+ Details

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

- Details

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

- 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:

- Details

Name:	srt_CounterpartyNameAxis=cmcsa_ATTInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmcsa_TimeWarnerInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments (Equity Method Investments) (Details) - USD ($)	1 Months Ended				3 Months Ended			9 Months Ended
	Aug. 31, 2019	May 31, 2019	Apr. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Apr. 30, 2019	Dec. 31, 2018	Aug. 31, 2016	Jul. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees, net					$ (355,000,000)		$ (76,000,000)	$ (295,000,000)	$ (56,000,000)
Equity method investments					5,363,000,000			5,363,000,000			$ 4,035,000,000
Investment securing collateralized obligation					816,000,000			816,000,000			0
Proceeds from collateralized obligation	$ 5,200,000,000							5,175,000,000	0
Collateralized obligation					5,165,000,000			5,165,000,000			0
Atairos [Member]
Schedule of Equity Method Investments [Line Items]
Pretax gain, sale of business to Atairos			$ 200,000,000
Atairos [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees, net					(262,000,000)		38,000,000	6,000,000	224,000,000
Cash capital contributions for equity method investments								475,000,000	133,000,000
Equity method investments					3,200,000,000			3,200,000,000			2,700,000,000
Hulu [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees, net					(101,000,000)		(132,000,000)	(351,000,000)	(370,000,000)
Equity method investments											248,000,000
Ownership percentage		33.00%								30.00%		30.00%	33.00%
Cash capital contributions for equity method investments		$ 477,000,000						903,000,000	341,000,000
Maximum total capital call in annual period		$ 1,500,000,000
Ownership percentage floor		21.00%
Fair market value assessment floor for put and call option		$ 27,500,000,000
Ownership percentage representing fair market value floor for put and call option		100.00%
Minimum guaranteed proceeds floor for put and call option		$ 5,800,000,000
Recognition of previously deferred gain						$ 159,000,000
Hulu [Member] \| AT&T Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		10.00%
Hulu [Member] \| Time Warner, Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage												10.00%
The Weather Channel [Member]
Schedule of Equity Method Investments [Line Items]
Pretax gain on sale of investment				$ 64,000,000
NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees, net					(88,000,000)		(119,000,000)	(299,000,000)	(306,000,000)
Equity method investments					1,290,000,000			1,290,000,000			707,000,000
Investment securing collateralized obligation					816,000,000			816,000,000			0
Proceeds from collateralized obligation	$ 5,200,000,000							5,175,000,000	0
Collateralized obligation					5,165,000,000			5,165,000,000			0
NBCUniversal Media LLC [Member] \| Hulu [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees, net					(101,000,000)		$ (132,000,000)	(351,000,000)	(370,000,000)
Equity method investments											$ 248,000,000
Ownership percentage		33.00%								30.00%		30.00%	33.00%
Cash capital contributions for equity method investments		$ 477,000,000						903,000,000	$ 341,000,000
Maximum total capital call in annual period		$ 1,500,000,000
Ownership percentage floor		21.00%
Fair market value assessment floor for put and call option		$ 27,500,000,000
Ownership percentage representing fair market value floor for put and call option		100.00%
Minimum guaranteed proceeds floor for put and call option		$ 5,800,000,000
Recognition of previously deferred gain						$ 159,000,000
NBCUniversal Media LLC [Member] \| Hulu [Member] \| AT&T Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		10.00%
NBCUniversal Media LLC [Member] \| Hulu [Member] \| Time Warner, Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage												10.00%
NBCUniversal Media LLC [Member] \| The Weather Channel [Member]
Schedule of Equity Method Investments [Line Items]
Pretax gain on sale of investment				$ 64,000,000
Level 2 [Member]
Schedule of Equity Method Investments [Line Items]
Estimated fair value of collateralized obligation					5,200,000,000			5,200,000,000
Level 2 [Member] \| NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Estimated fair value of collateralized obligation					$ 5,200,000,000			$ 5,200,000,000