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

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