v2.4.1.9

Equity (Narrative) (Details) (USD $) In Billions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014
Number of votes entitled of each class B common stock | 15 cmcsa_NumberOfVotesEntitledOfEachClassBCommonStock
Availability remaining under share repurchase program | $ 3.25 us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount
Existing Authorization [Member] |
Share repurchase authorization | 7.5 us-gaap_StockRepurchaseProgramAuthorizedAmount / us-gaap_ShareRepurchaseProgramAxis = cmcsa_ExistingAuthorizationMember
New Authorization [Member] |
Share repurchase authorization | $ 10.0 us-gaap_StockRepurchaseProgramAuthorizedAmount / us-gaap_ShareRepurchaseProgramAxis = cmcsa_NewAuthorizationMember
Class A Common Stock [Member] |
Voting power | 66.67%cmcsa_CommonStockVotingPower / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember
Class B Common Stock [Member] |
Voting power | 33.33%cmcsa_CommonStockVotingPower / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember

X
- Definition Common Stock Voting Power + References No definition available. + Details
Name: | cmcsa_CommonStockVotingPower
Namespace Prefix: | cmcsa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of votes entitled to vote for each share of Class B common stock share. + References No definition available. + Details
Name: | cmcsa_NumberOfVotesEntitledOfEachClassBCommonStock
Namespace Prefix: | cmcsa_
Data Type: | xbrli:positiveIntegerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of a stock repurchase plan authorized by an entity's Board of Directors. + References No definition available. + Details
Name: | us-gaap_StockRepurchaseProgramAuthorizedAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount remaining of a stock repurchase plan authorized by an entity's Board of Directors. + References No definition available. + Details
Name: | us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_ShareRepurchaseProgramAxis=cmcsa_ExistingAuthorizationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ShareRepurchaseProgramAxis=cmcsa_NewAuthorizationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassBMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |