v2.4.1.9

Income Taxes (Components of Net Deferred Tax Liability) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Net operating loss carryforwards | $ 448 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards | $ 495 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
Differences between book and tax basis of long-term debt | 116 us-gaap_DeferredTaxAssetsOther | 117 us-gaap_DeferredTaxAssetsOther
Nondeductible accruals and other | 3,383 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsOther | 3,588 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsOther
Less: Valuation allowance | 375 us-gaap_DeferredTaxAssetsValuationAllowance | 405 us-gaap_DeferredTaxAssetsValuationAllowance
Total deferred tax assets | 3,572 us-gaap_DeferredTaxAssetsNet | 3,795 us-gaap_DeferredTaxAssetsNet
Total deferred tax liabilities | 36,336 us-gaap_DeferredIncomeTaxLiabilities | 35,494 us-gaap_DeferredIncomeTaxLiabilities
Net deferred tax liability | 32,764 us-gaap_DeferredTaxLiabilities | 31,699 us-gaap_DeferredTaxLiabilities
Property And Equipment And Intangible Assets [Member] |  |
Deferred tax liabilities, other | 35,112 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = cmcsa_PropertyAndEquipmentAndIntangibleAssetsMember | 34,044 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = cmcsa_PropertyAndEquipmentAndIntangibleAssetsMember
Investments [Member] |  |
Deferred tax liabilities, other | 186 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = us-gaap_InvestmentsMember | 473 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = us-gaap_InvestmentsMember
Indexed Debt Securities [Member] |  |
Deferred tax liabilities, other | 534 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = cmcsa_IndexedDebtSecuritiesMember | 610 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = cmcsa_IndexedDebtSecuritiesMember
Foreign Subsidiaries And Undistributed Foreign Earnings [Member] |  |
Deferred tax liabilities, other | $ 504 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = cmcsa_ForeignSubsidiariesAndUndistributedForeignEarningsMember | $ 367 us-gaap_DeferredTaxLiabilitiesOther / cmcsa_TaxAdjustmentsAxis = cmcsa_ForeignSubsidiariesAndUndistributedForeignEarningsMember

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