v2.4.1.9

Fair Value Disclosure Rollforward (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
NBCUniversal Enterprise [Member]
Fair value of redeemable preferred stock	$ 764cmcsa_FairValueAmountOfRedeemablePreferredShares / dei_LegalEntityAxis = cmcsa_NbcuniversalEnterpriseMember
Contractual Obligations [Member]
Beginning balance	883us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember
Fair value adjustments	40us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember
Payments	(19)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember
Ending balance	904us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember
Contractual Obligations [Member] | NBCUniversal Media, LLC [Member]
Beginning balance	883us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Fair value adjustments	40us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Payments	(19)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Ending balance	904us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Contingent Consideration [Member]
Beginning balance	644us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember
Fair value adjustments	7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember
Payments	(19)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember
Ending balance	$ 632us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember
Maximum [Member]
Discount Rate Used To Determine Fair Value	13.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MaximumMember
Maximum [Member] | NBCUniversal Media, LLC [Member]
Discount Rate Used To Determine Fair Value	13.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MaximumMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Minimum [Member]
Discount Rate Used To Determine Fair Value	5.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MinimumMember
Minimum [Member] | NBCUniversal Media, LLC [Member]
Discount Rate Used To Determine Fair Value	12.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MinimumMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember

X

- Definition

Fair value of our redeemable subsidiary preferred shares and/or stock

+ References

No definition available.

+ Details

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

X

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ 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 (bbb)

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

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X

- Definition

Amount of increase (decrease) of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)

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

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X

- Definition

Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)(2)

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

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Balance Type:	debit
Period Type:	duration

X

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)

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

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- Details

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X

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X

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X

- Details

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