v2.4.1.9

Income Taxes (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 19, 2013
Foreign income before taxes		$ 385,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	$ 524,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign	$ 434,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign
Liability for uncertain tax positions, noncurrent		1,200,000,000us-gaap_LiabilityForUncertainTaxPositionsNoncurrent
Unrecognized tax benefits that would impact effective tax rate		603,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Unrecognized tax benefits, interest on income taxes accrued		452,000,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued	780,000,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued
Change In Income Tax Expense Related To Effectively Settled Uncertain Tax Positions	724,000,000cmcsa_ChangeInIncomeTaxExpenseRelatedToEffectivelySettledUncertainTaxPositions
Income Tax Effects Allocated Directly to Equity, Other		299,000,000us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityOther	244,000,000us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityOther	164,000,000us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityOther
Deferred income taxes, total		32,959,000,000us-gaap_DeferredTaxLiabilitiesNoncurrent	31,935,000,000us-gaap_DeferredTaxLiabilitiesNoncurrent
Franchise Rights [Member]
Net deferred tax liability, franchise rights		23,000,000,000cmcsa_DeferredTaxLiabilityFranchiseRights / dei_LegalEntityAxis = us-gaap_FranchiseRightsMember
Uncertain Tax Positions Indemnified By GE [Member]
Liability for uncertain tax positions, noncurrent		243,000,000us-gaap_LiabilityForUncertainTaxPositionsNoncurrent / dei_LegalEntityAxis = cmcsa_UncertainTaxPositionsIndemnifiedByGeMember
Unrecognized tax benefits, interest on income taxes accrued		44,000,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued / dei_LegalEntityAxis = cmcsa_UncertainTaxPositionsIndemnifiedByGeMember	42,000,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued / dei_LegalEntityAxis = cmcsa_UncertainTaxPositionsIndemnifiedByGeMember
Redemption Transaction [Member]
Deferred income taxes, total					1,600,000,000us-gaap_DeferredTaxLiabilitiesNoncurrent / dei_LegalEntityAxis = cmcsa_RedemptionTransactionMember
Other Acquisitions [Member]
Business Acquisition Purchase Price Allocation Deferred Income Taxes Asset Liability Net1		26,000,000cmcsa_BusinessAcquisitionPurchasePriceAllocationDeferredIncomeTaxesAssetLiabilityNet1 / dei_LegalEntityAxis = cmcsa_OtherAcquisitionsMember
Other Comprehensive Income (Loss) [Member]
Income Tax Effects Allocated Directly to Equity, Other		127,000,000us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityOther / cmcsa_TaxAdjustmentsAxis = us-gaap_OtherComprehensiveIncomeMember
Domestic Tax Authority
Operating loss carryforwards		127,000,000us-gaap_OperatingLossCarryforwards / cmcsa_TaxAdjustmentsAxis = us-gaap_DomesticCountryMember
Operating loss carryforwards expiration dates		Dec. 31, 2034
Foreign Tax Authority
Operating loss carryforwards		$ 331,000,000us-gaap_OperatingLossCarryforwards / cmcsa_TaxAdjustmentsAxis = us-gaap_ForeignCountryMember
Operating loss carryforwards expiration dates		Dec. 31, 2024

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