v2.4.1.9

Investments (Fair Value Method Investments) (Details) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair value equity securities held as collateral	$ 523,000,000cmcsa_EquitySecuritiesHeldAsCollateral	$ 3,959,000,000cmcsa_EquitySecuritiesHeldAsCollateral
Obligations under prepaid forward sale agreements	119,000,000cmcsa_ObligationsUnderPrepaidForwardSaleAgreements	811,000,000cmcsa_ObligationsUnderPrepaidForwardSaleAgreements
Derivative component of prepaid forward sale agreements	356,000,000cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreements	2,800,000,000cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreements
Total liabilities associated with prepaid forward sale agreements	475,000,000cmcsa_LiabilitiesRelatedToPrepaidForwardSaleAgreements	3,611,000,000cmcsa_LiabilitiesRelatedToPrepaidForwardSaleAgreements
Fair Value Equity Securities Settlement	3,200,000,000cmcsa_FairValueEquitySecuritiesSettlement	1,400,000,000cmcsa_FairValueEquitySecuritiesSettlement
Proceeds From Sales Of Businesses And Investments	666,000,000cmcsa_ProceedsFromSalesOfBusinessesAndInvestments	1,083,000,000cmcsa_ProceedsFromSalesOfBusinessesAndInvestments	3,102,000,000cmcsa_ProceedsFromSalesOfBusinessesAndInvestments
Clearwire LLC [Member]
Proceeds from sale of available-for-sale securities		443,000,000us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities / dei_LegalEntityAxis = cmcsa_ClearwireLlcMember
Fair Value Investment Shares Sold		89cmcsa_FairValueInvestmentSharesSold / dei_LegalEntityAxis = cmcsa_ClearwireLlcMember
Pre-tax gain realized on Clearwire		443,000,000us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax / dei_LegalEntityAxis = cmcsa_ClearwireLlcMember
Liberty Media Redemption Transaction [Member]
Shares Redeemed		6.3cmcsa_SharesRedeemed / dei_LegalEntityAxis = cmcsa_LibertyMediaRedemptionTransactionMember
Fair value of the shares delivered in the Liberty Media exchange transaction		937,000,000cmcsa_FairValueOfSharesDeliveredInTransaction / dei_LegalEntityAxis = cmcsa_LibertyMediaRedemptionTransactionMember
Proceeds From Sales Of Businesses And Investments		$ 417,000,000cmcsa_ProceedsFromSalesOfBusinessesAndInvestments / dei_LegalEntityAxis = cmcsa_LibertyMediaRedemptionTransactionMember

X

- Definition

This element represents the fair value as of the balance sheet date of embedded derivative liabilities as a component of prepaid forward sale agreements valued on a recurring basis.

+ References

No definition available.

+ Details

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

X

- Definition

Fair value method equity securities held as collateral related to our obligations under prepaid forward sale agreements.

+ References

No definition available.

+ Details

Name:	cmcsa_EquitySecuritiesHeldAsCollateral
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Fair value method equity securities used to settle our obligations under prepaid forward sale agreements in the period.

+ References

No definition available.

+ Details

Name:	cmcsa_FairValueEquitySecuritiesSettlement
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Represents the amount of Clearwire shares sold to a third party.

+ References

No definition available.

+ Details

Name:	cmcsa_FairValueInvestmentSharesSold
Namespace Prefix:	cmcsa_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the fair value of the shares delivered in the Liberty Media exchange transaction.

+ References

No definition available.

+ Details

Name:	cmcsa_FairValueOfSharesDeliveredInTransaction
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

The carrying value of the liabilities related to prepaid forward sale agreements.

+ References

No definition available.

+ Details

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

X

- Definition

The carrying value of obligations related to prepaid forward sale agreements.

+ References

No definition available.

+ Details

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

X

- Definition

The cash inflow associated with the sale of businesses and investments during the period.

+ References

No definition available.

+ Details

Name:	cmcsa_ProceedsFromSalesOfBusinessesAndInvestments
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Represents the number of shares held by the Company as a fair value investment which were redeemed by the issuer.

+ References

No definition available.

+ Details

Name:	cmcsa_SharesRedeemed
Namespace Prefix:	cmcsa_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Amount before tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e689-108580

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26853-111562



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 12

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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

X

- Details

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