v2.4.1.9

Pension Plans and Postretirement Benefits (Benefit Obligation and Expense for Our Split-Dollar Life Insurance Agreements) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Postretirement and Pension Plans |  |  |
Benefit obligation | $ 217 cmcsa_SplitDollarLifeInsuranceAgreementsBenefitObligation | $ 212 cmcsa_SplitDollarLifeInsuranceAgreementsBenefitObligation | $ 202 cmcsa_SplitDollarLifeInsuranceAgreementsBenefitObligation
Other operating and administrative expenses | $ 52 cmcsa_SplitDollarLifeInsuranceAgreementExpense | $ 50 cmcsa_SplitDollarLifeInsuranceAgreementExpense | $ 58 cmcsa_SplitDollarLifeInsuranceAgreementExpense

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- Definition Split Dollar Life Insurance Agreement Expense + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Split-dollar life insurance agreements benefit obligation + References No definition available. + Details
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Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration