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Condensed Consolidating Financial Information (Narrative) (Details) $ in Millions	Jun. 30, 2018 USD ($)
NBCUniversal Enterprise Senior Debt Securities [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee	$ 3,000
NBCUniversal Enterprise Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility subject to guarantee	1,500
Universal Studios Japan Term Loan Maturing March 2022 [Member]
Debt Instrument [Line Items]
Term loans subject to guarantee	3,600
Comcast Holdings' ZONES due October 2029 [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee	185
Comcast Holdings' ZONES due November 2029 [Member]
Debt Instrument [Line Items]
Principal amount of debt securities not subject to guarantee	$ 62

X

- Definition

Principal amount of debt securities subject to guarantee.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

X

- Definition

Guarantee Obligations, Line Of Credit Facility

+ References

No definition available.

+ Details

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

X

- Definition

Guarantee Obligations, Term Loans

+ References

No definition available.

+ Details

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

X

- Definition

Principal amount of debt instruments not guaranteed by Comcast Corporation, the cable guarantors nor NBCUniversal Media, LLC.

+ References

No definition available.

+ Details

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Namespace Prefix:	cmcsa_
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Balance Type:	credit
Period Type:	instant

X

- 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

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- Details

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X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_NbcuniversalEnterpriseCreditFacilityMember
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Balance Type:
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X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_UniversalStudiosJapanTermLoanMaturingMarch2022Member
Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_ComcastHoldingsZonesDueOctober2029Member
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Balance Type:
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X

- Details

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