Amount represents the maximum borrowing capacity under the commercial paper program.

No definition available.

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

- Definition

Principal amount of debt securities subject to guarantee.

+ References

No definition available.

+ Details

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

- Definition

Potential increased maximum borrowing capacity under the revolving credit facility, subject to approval of the lenders.

+ References

No definition available.

+ Details

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

- Definition

Represents the Company's ownership in its cable holding company subsidiaries.

+ References

No definition available.

+ Details

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

- Definition

Represents the related party aggregate principal amount of senior notes not subject to guarantee by NBCUniversal.

+ References

No definition available.

+ Details

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

- Definition

Represents the related party credit facility not subject to guarantee by NBCUniversal.

+ References

No definition available.

+ Details

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

- Definition

Represents the related party liquidation preference of preferred stock not subject to guarantee by NBCUniversal.

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_CounterpartyNameAxis=cmcsa_NbcuniversalEnterpriseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_ComcastrevolvingcreditfacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_NbcuniversalEnterpriseCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_NbcuniversalEnterpriseCommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_ComcastAndComcastCableDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt (Narrative) (Details) - USD ($)	12 Months Ended
Long-Term Debt (Narrative) (Details) - USD ($)	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Fair value of debt	$ 71,700,000,000		$ 66,300,000,000
Amounts available under revolving credit facilities	7,400,000,000
Unused irrevocable standby letters of credit	439,000,000
NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Fair value of debt	$ 13,500,000,000		12,600,000,000
Ownership in cable holding company subsidiaries	100.00%
NBCUniversal Enterprise [Member]
Debt Instrument [Line Items]
Related party liquidation preference preferred stock not subject to guarantee	$ 725,000,000
NBCUniversal Enterprise [Member] \| NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Related party aggregate principal amount senior notes not subject to guarantee	4,800,000,000
Related party credit facility not subject to guarantee	1,500,000,000
Related party liquidation preference preferred stock not subject to guarantee	$ 725,000,000
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Borrowing margin for LIBOR based borrowings	1.00%
Comcast Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 7,000,000,000		7,000,000,000
Revolving credit facility amount outstanding	0		0
Potential increase of maximum borrowing capacity	10,000,000,000
Comcast Revolving Credit Facility [Member] \| NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee	7,000,000,000
NBCUniversal Enterprise Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	1,500,000,000		1,500,000,000
Revolving credit facility amount outstanding	0		$ 0
Potential increase of maximum borrowing capacity	2,000,000,000
Amounts available under revolving credit facilities	1,500,000,000
Commercial Paper [Member]
Debt Instrument [Line Items]
Commercial paper capacity		$ 7,000,000,000
NBCUniversal Enterprise Commercial Paper [Member]
Debt Instrument [Line Items]
Commercial paper capacity		$ 1,500,000,000
Comcast And Comcast Cable Debt Securities [Member] \| NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee	$ 49,100,000,000