v2.4.1.9

Related Party Transactions (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Borrowing margin for LIBOR based borrowings	1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Settlement Of Station Venture Liability	$ 0cmcsa_SettlementOfStationVentureLiability	$ 602,000,000cmcsa_SettlementOfStationVentureLiability	$ 0cmcsa_SettlementOfStationVentureLiability
Other noncurrent liabilities	10,819,000,000us-gaap_OtherLiabilitiesNoncurrent	11,384,000,000us-gaap_OtherLiabilitiesNoncurrent
NBCUniversal Media, LLC [Member]
Revolving Credit Agreement Capacity		3,000,000,000cmcsa_RevolvingCreditAgreementCapacity / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Collateral Liability For Unconsolidated Subsidiary Debt		482,000,000cmcsa_CollateralLiabilityForUnconsolidatedSubsidiaryDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Borrowing margin for LIBOR based borrowings	1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Settlement Of Station Venture Liability	0cmcsa_SettlementOfStationVentureLiability / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	602,000,000cmcsa_SettlementOfStationVentureLiability / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	0cmcsa_SettlementOfStationVentureLiability / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Redemption transaction distribution	0cmcsa_RedemptionTransactionDistribution / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	3,200,000,000cmcsa_RedemptionTransactionDistribution / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	0cmcsa_RedemptionTransactionDistribution / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Other noncurrent liabilities	3,722,000,000us-gaap_OtherLiabilitiesNoncurrent / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	3,412,000,000us-gaap_OtherLiabilitiesNoncurrent / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Nbcuniversal Holdings Preferred Stock [Member] | NBCUniversal Media, LLC [Member]
Preferred stock liquidition preference value	9,400,000,000us-gaap_PreferredStockLiquidationPreferenceValue / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cmcsa_NbcuniversalHoldingsPreferredStockMember
Preferred stock dividend rate percentage	8.25%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cmcsa_NbcuniversalHoldingsPreferredStockMember
Carrying Value Of Related Party Contractual Obligation [Member] | NBCUniversal Media, LLC [Member]
Other noncurrent liabilities	$ 383,000,000us-gaap_OtherLiabilitiesNoncurrent / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cmcsa_CarryingValueOfRelatedPartyContractualObligationMember	$ 383,000,000us-gaap_OtherLiabilitiesNoncurrent / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cmcsa_CarryingValueOfRelatedPartyContractualObligationMember

X

- Definition

Collateral Liability For Unconsolidated Subsidiary Debt

+ References

No definition available.

+ Details

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

X

- Definition

The cash distribution to NBCUniversal Holdings associated with the redemption transaction.

+ References

No definition available.

+ Details

Name:	cmcsa_RedemptionTransactionDistribution
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The maximum amount of borrowing or lending capacity with respect to the cash management arrangement between Comcast and NBCUniversal under the revolving credit agreement.

+ References

No definition available.

+ Details

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

X

- Definition

The cash outflow associated with the settlement of the Station Venture liability.

+ References

No definition available.

+ Details

Name:	cmcsa_SettlementOfStationVentureLiability
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

Aggregate carrying amount, as of the balance sheet date, of noncurrent obligations not separately disclosed in the balance sheet. Noncurrent liabilities are expected to be paid after one year (or the normal operating cycle, if longer).

+ 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 210.5-02.24)

-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 24

-Article 5

+ Details

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

X

- Definition

The percentage rate used to calculate dividend payments on preferred stock.

+ References

No definition available.

+ Details

Name:	us-gaap_PreferredStockDividendRatePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

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

X

- Details

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

X

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=cmcsa_CarryingValueOfRelatedPartyContractualObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=cmcsa_NbcuniversalHoldingsPreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: