v2.4.1.9

Share-Based Compensation (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Number of stock options granted	17.6cmcsa_StockOptionsGranted
Number of restricted share units granted	5.1cmcsa_RestrictedShareUnitsGranted
Weighted average fair value of stock options granted	$ 11.79cmcsa_FairValueOfStockOptionsGranted
Weighted average fair value of restricted share units granted	$ 59.5cmcsa_FairValueOfRestrictedShareUnitsGranted
Stock Options [Member]
Unrecognized pretax compensation expense on nonvested awards	$ 461us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EmployeeStockOptionMember
Restricted Share Units [Member]
Unrecognized pretax compensation expense on nonvested awards	$ 725us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_RestrictedStockMember

X

- Definition

The weighted average fair value of restricted share units granted related to our annual management grant program.

+ References

No definition available.

+ Details

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Namespace Prefix:	cmcsa_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The weighted average fair value of stock options granted related to our annual management grant program.

+ References

No definition available.

+ Details

Name:	cmcsa_FairValueOfStockOptionsGranted
Namespace Prefix:	cmcsa_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of restricted share units granted related to our annual management grant program.

+ References

No definition available.

+ Details

Name:	cmcsa_RestrictedShareUnitsGranted
Namespace Prefix:	cmcsa_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of stock options granted related to our annual management grant program.

+ References

No definition available.

+ Details

Name:	cmcsa_StockOptionsGranted
Namespace Prefix:	cmcsa_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: