v2.4.1.9

Long-Term Debt (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Long-term Debt, Fair Value	$ 55,300,000,000us-gaap_LongTermDebtFairValue	$ 51,800,000,000us-gaap_LongTermDebtFairValue
Borrowing margin for LIBOR based borrowings	1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Amounts available under revolving credit facilities	6,500,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Unused irrevocable standby letters of credit	473,000,000us-gaap_LettersOfCreditOutstandingAmount
Commercial paper	845,000,000us-gaap_CommercialPaper	1,350,000,000us-gaap_CommercialPaper
NBCUniversal Media, LLC [Member]
Long-term Debt, Fair Value	11,500,000,000us-gaap_LongTermDebtFairValue / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	11,700,000,000us-gaap_LongTermDebtFairValue / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Comcast And Comcast Cable Communications LLC [Member]
Debt Instrument, Maturity Date, Description	June 2017
Credit facility borrowing capacity	6,250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / dei_LegalEntityAxis = cmcsa_ComcastAndComcastCableCommunicationsLlcMember
NBCUniversal Enterprise [Member] | NBCUniversal Media, LLC [Member]
Principal amount	4,000,000,000us-gaap_DebtInstrumentFaceAmount / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseMember
Preferred stock liquidition preference value	725,000,000us-gaap_PreferredStockLiquidationPreferenceValue / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseMember
NBCUniversal Enterprise Commercial Paper [Member]
Credit facility borrowing capacity	1,350,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseCommercialPaperMember
Comcast And Comcast Cable Revolving Credit Facility [Member] | NBCUniversal Media, LLC [Member]
Guarantee Obligations Current Principal Balance	6,250,000,000cmcsa_GuaranteeObligationsCurrentPrincipalBalance / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_ComcastAndComcastCableRevolvingCreditFacilityMember
Debt Instrument, Maturity Date, Description	June 2017
Comcast And Comcast Cable Debt Securities [Member] | NBCUniversal Media, LLC [Member]
Guarantee Obligations Current Principal Balance	32,600,000,000cmcsa_GuaranteeObligationsCurrentPrincipalBalance / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_ComcastAndComcastCableDebtSecuritiesMember
NBCUniversal Enterprise Credit Facility [Member]
Debt Instrument, Maturity Date, Description	March 2018
Borrowing margin for LIBOR based borrowings	1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseCreditFacilityMember
Credit facility borrowing capacity	1,350,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseCreditFacilityMember
Amounts available under revolving credit facilities	505,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseCreditFacilityMember
NBCUniversal Enterprise Credit Facility [Member] | NBCUniversal Media, LLC [Member]
Credit facility borrowing capacity	$ 1,350,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseCreditFacilityMember

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

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 7



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(a)(3))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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 210.9-03.13(2))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

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-Subparagraph a(3)

-Article 5



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13

-Subparagraph 2

-Article 9

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

X

- 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

X

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

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

X

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: 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(a)(2))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

X

- 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

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 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

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

+ 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 210.5-02.19(b),22(b))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
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X

- 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
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Value of the difference between preference in liquidation and the par or stated values of the preferred shares.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

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X

- Details

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X

- Details

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X

- Details

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Data Type:	na
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Period Type:

X

- Details

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