Long-Term Debt Long-term Debt (Cross-Guarantee Structure) (Details) $ in Millions	Jun. 30, 2017 USD ($)
NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Ownership in cable holding company subsidiaries	100.00%
NBCUniversal Enterprise [Member]
Debt Instrument [Line Items]
Related party aggregate principal amount senior notes not subject to guarantee	$ 4,800
NBCUniversal Enterprise [Member]
Debt Instrument [Line Items]
Related party credit facility not subject to guarantee	1,500
Related party liquidation preference preferred stock not subject to guarantee	725
Comcast Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Guarantee obligations, maximum capacity	7,000
Comcast and Comcast Cable Debt Securities [Member]
Debt Instrument [Line Items]
Guarantee Obligations Current Principal Balance	$ 46,100

Principal amount of debt securities subject to guarantee.

No definition available.

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

- Definition

Maximum capacity of credit facility subject to cross-guarantee

+ References

No definition available.

+ Details

Name:	cmcsa_GuaranteeObligationsMaximumCapacityOfCreditFacility
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

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

- Details

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

- Details

Name:	dei_LegalEntityAxis=cmcsa_NbcuniversalEnterpriseMember
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_DebtInstrumentAxis=cmcsa_ComcastrevolvingcreditfacilityMember
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: