v2.4.1.9

Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Trading securities	$ 42us-gaap_TradingSecurities	$ 523us-gaap_TradingSecurities
Available-for-sale securities	132us-gaap_AvailableForSaleSecurities	132us-gaap_AvailableForSaleSecurities
Interest rate swap agreements	91us-gaap_InterestRateDerivativeAssetsAtFairValue	84us-gaap_InterestRateDerivativeAssetsAtFairValue
Other [assets]	56us-gaap_OtherAssetsFairValueDisclosure	71us-gaap_OtherAssetsFairValueDisclosure
Fair Value, Assets Measured on Recurring Basis, Total	321us-gaap_AssetsFairValueDisclosure	810us-gaap_AssetsFairValueDisclosure
Derivative component of prepaid forward sale agreements and indexed debt instruments	3cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreementsAndIndexedDebtInstruments	361cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreementsAndIndexedDebtInstruments
Contractual obligations	904us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations	883us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations
Contingent consideration	632us-gaap_BusinessCombinationContingentConsiderationLiability	644us-gaap_BusinessCombinationContingentConsiderationLiability
Other [liabilities]	50us-gaap_OtherLiabilitiesFairValueDisclosure	8us-gaap_OtherLiabilitiesFairValueDisclosure
Fair Value Liabilities Measured On Recurring Basis, Total	1,589us-gaap_LiabilitiesFairValueDisclosure	1,896us-gaap_LiabilitiesFairValueDisclosure
Fair Value, Inputs, Level 1 [Member]
Trading securities	42us-gaap_TradingSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Available-for-sale securities	1us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Interest rate swap agreements	0us-gaap_InterestRateDerivativeAssetsAtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Other [assets]	0us-gaap_OtherAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Assets Measured on Recurring Basis, Total	43us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Derivative component of prepaid forward sale agreements and indexed debt instruments	0cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreementsAndIndexedDebtInstruments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Contractual obligations	0us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Contingent consideration	0us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Other [liabilities]	0us-gaap_OtherLiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value Liabilities Measured On Recurring Basis, Total	0us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 2 [Member]
Trading securities	0us-gaap_TradingSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Available-for-sale securities	121us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Interest rate swap agreements	91us-gaap_InterestRateDerivativeAssetsAtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Other [assets]	48us-gaap_OtherAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Assets Measured on Recurring Basis, Total	260us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Derivative component of prepaid forward sale agreements and indexed debt instruments	3cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreementsAndIndexedDebtInstruments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Contractual obligations	0us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Contingent consideration	0us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Other [liabilities]	50us-gaap_OtherLiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value Liabilities Measured On Recurring Basis, Total	53us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 3 [Member]
Trading securities	0us-gaap_TradingSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Available-for-sale securities	10us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Interest rate swap agreements	0us-gaap_InterestRateDerivativeAssetsAtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Other [assets]	8us-gaap_OtherAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Fair Value, Assets Measured on Recurring Basis, Total	18us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Derivative component of prepaid forward sale agreements and indexed debt instruments	0cmcsa_DerivativeFairValueOfEmbeddedDerivativeLiabilityComponentOfPrepaidForwardSaleAgreementsAndIndexedDebtInstruments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Contractual obligations	904us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Contingent consideration	632us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Other [liabilities]	0us-gaap_OtherLiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Fair Value Liabilities Measured On Recurring Basis, Total	$ 1,536us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member

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 and indexed debt instruments valued on a recurring basis.

+ References

No definition available.

+ Details

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X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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X

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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Period Type:	instant

X

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476



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-Section 35

-Paragraph 1

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X

- Definition

Fair value of obligations measured on a recurring basis.

+ References

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-SubTopic 10

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-Paragraph 2

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X

- Definition

Fair value as of the balance sheet date of interest rate derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets.

+ References

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-Publisher FASB

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-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611

+ Details

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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-Topic 820

-SubTopic 10

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-Paragraph 2

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- Definition

Fair value portion of other assets.

+ References

No definition available.

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- Definition

Fair value portion of other liabilities.

+ References

No definition available.

+ Details

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

- Definition

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

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