v2.4.1.9

Pension Plans and Postretirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Severance costs	$ 152us-gaap_SeveranceCosts1	$ 160us-gaap_SeveranceCosts1	$ 155us-gaap_SeveranceCosts1
Multiemployer Plans Funded Status	At least 80 percent
NBCUniversal Media, LLC [Member]
Severance costs	113us-gaap_SeveranceCosts1 / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	116us-gaap_SeveranceCosts1 / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	90us-gaap_SeveranceCosts1 / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Multiemployer Plans Funded Status	At least 80 percent
Postretirement Benefits [Member]
Plan funded status	(837)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember	(633)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember	(703)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Postretirement Benefits [Member] | NBCUniversal Media, LLC [Member]
Plan funded status	(209)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	(158)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	(177)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Pension Benefits [Member]
Multiemployer plan contributions	58us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember	59us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember	40us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Plan funded status	(339)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember	(278)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember	(402)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Contribution to fully fund and settle plan		55us-gaap_DefinedBenefitPlanSettlementsPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Reclassification adjustment from accumulated other comprehensive income before tax		74us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Pension Benefits [Member] | NBCUniversal Media, LLC [Member]
Multiemployer plan contributions	58us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	59us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	40us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Plan funded status	(339)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	(278)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	(329)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Deferred Compensation Plans [Member]
Cash surrender value of life insurance policies	628us-gaap_CashSurrenderValueOfLifeInsurance / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_DeferredCompensationPlansMember	565us-gaap_CashSurrenderValueOfLifeInsurance / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_DeferredCompensationPlansMember
Retirement Investment Plans [Member]
Expenses related to retirement investment plans	379us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_RetirementInvestmentPlansMember	324us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_RetirementInvestmentPlansMember	246us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_RetirementInvestmentPlansMember
Retirement Investment Plans [Member] | NBCUniversal Media, LLC [Member]
Expenses related to retirement investment plans	165us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_RetirementInvestmentPlansMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	152us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_RetirementInvestmentPlansMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	85us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_RetirementInvestmentPlansMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
NBCUniversal Qualified Pension Plan [Member]
Plan funded status	16us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_NbcuniversalQualifiedPensionPlanMember
NBCUniversal Qualified Pension Plan [Member] | NBCUniversal Media, LLC [Member]
Plan funded status	16us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_NbcuniversalQualifiedPensionPlanMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Benefit obligations in excess of plan assets	51us-gaap_DefinedBenefitPlanPlansWithBenefitObligationsInExcessOfPlanAssetsAggregateFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_NbcuniversalQualifiedPensionPlanMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	43us-gaap_DefinedBenefitPlanPlansWithBenefitObligationsInExcessOfPlanAssetsAggregateFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_NbcuniversalQualifiedPensionPlanMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Postretirement and Other Benefits [Member]
Multiemployer plan contributions	125us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_PostretirementAndOtherBenefitsMember	98us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_PostretirementAndOtherBenefitsMember	114us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_PostretirementAndOtherBenefitsMember
Postretirement and Other Benefits [Member] | NBCUniversal Media, LLC [Member]
Multiemployer plan contributions	$ 125us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_PostretirementAndOtherBenefitsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	$ 98us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_PostretirementAndOtherBenefitsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	$ 114us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cmcsa_PostretirementAndOtherBenefitsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember

X

- Definition

Carrying amount as of the balance sheet date of amounts which could be received based on the terms of the insurance contract upon surrendering life policies owned by the entity.

+ 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 210.5-02.17)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section 45

-Paragraph 4

-Subparagraph (d)

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

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- Definition

If aggregate disclosures are presented, the aggregate fair value of plan assets for plans with benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 3

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- Definition

The amount that relates to an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include (a) making lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and (b) purchasing nonparticipating annuity contracts.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Section 15

-Paragraph 6

-Subparagraph (a)(1)

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

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- Definition

Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

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

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- Definition

Indicate whether the multiemployer plan is less than 65 percent funded, between 65 and less than 80 percent funded, or at least 80 percent funded. The acceptable values are: (1) "less than 65 percent", (2) "between 65 and less than 80 percent", (3) "at least 80 percent", and (4) "NA". A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

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

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- Definition

Amount before tax of reclassification adjustment from accumulated other comprehensive (income) loss related to net period benefit cost (credit) for pension and other postretirement defined benefit plans.

+ References

No definition available.

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- Definition

Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section P

-Subsection 3, 4



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