v2.4.1.9

Significant Transactions (Narrative) (Details) (Transaction-Related Expenses Member, USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Transaction-Related Expenses Member |  |
Significant Transactions [Line Items] |  |
Other operating and administrative expenses | $ 99 cmcsa_Transactioncostsandtransactionrelatedcostsinoperatingcostsandexpenses / us-gaap_BusinessAcquisitionAxis = cmcsa_TransactionRelatedExpensesMember | $ 17 cmcsa_Transactioncostsandtransactionrelatedcostsinoperatingcostsandexpenses / us-gaap_BusinessAcquisitionAxis = cmcsa_TransactionRelatedExpensesMember

X
- Details
Name: | cmcsa_SignificantTransactionsLineItems
Namespace Prefix: | cmcsa_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Transaction costs and transaction related costs in other operating costs and administrative expenses + References No definition available. + Details
Name: | cmcsa_Transactioncostsandtransactionrelatedcostsinoperatingcostsandexpenses
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration