v2.4.1.9

Commitments and Contingencies (Rental expense) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Rent expense | $ 580 us-gaap_LeaseAndRentalExpense | $ 616 us-gaap_LeaseAndRentalExpense | $ 688 us-gaap_LeaseAndRentalExpense
NBCUniversal Media, LLC [Member] |  |  |
Rent expense | $ 222 us-gaap_LeaseAndRentalExpense / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | $ 250 us-gaap_LeaseAndRentalExpense / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | $ 317 us-gaap_LeaseAndRentalExpense / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember

X
- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
Name: | us-gaap_LeaseAndRentalExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=cmcsa_NBCUniversalMediaLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |