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Long-Term Debt (Cross-Guarantee Structure) (Details) - NBCUniversal Media LLC [Member] $ in Millions	Jun. 30, 2018 USD ($)
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	$ 3,000
Related party credit facility not subject to guarantee	1,500
Related party liquidation preference preferred stock not subject to guarantee	725
Comcast and Comcast Cable Debt Securities [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee	50,300
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Guarantee obligations, maximum capacity	$ 7,000

X

- Definition

Principal amount of debt securities subject to guarantee.

+ References

No definition available.

+ Details

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

X

- Definition

Maximum capacity of credit facility subject to cross-guarantee

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
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X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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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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X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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

X

- Details

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