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	15cmcsa_NumberOfVotesEntitledOfEachClassBCommonStock
Availability remaining under share repurchase program	$ 3.25us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount
Existing Authorization [Member]
Share repurchase authorization	7.5us-gaap_StockRepurchaseProgramAuthorizedAmount / us-gaap_ShareRepurchaseProgramAxis = cmcsa_ExistingAuthorizationMember
New Authorization [Member]
Share repurchase authorization	$ 10.0us-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: