v2.4.1.9

Investments (Equity Method Investments) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Impairment of investment		$ 249,000,000us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Return of capital from investees	25,000,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	149,000,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	2,362,000,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital
NBCUniversal Media, LLC [Member]
Impairment of investment		249,000,000us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
The Weather Channel [Member]
Total distribution from investee		152,000,000cmcsa_TotalDistributionFromInvestee / dei_LegalEntityAxis = cmcsa_WeatherChannelMember
Return of capital from investees		128,000,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital / dei_LegalEntityAxis = cmcsa_WeatherChannelMember
The Weather Channel [Member] | NBCUniversal Media, LLC [Member]
Total distribution from investee		152,000,000cmcsa_TotalDistributionFromInvestee / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_WeatherChannelMember
Return of capital from investees		128,000,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_WeatherChannelMember
Hulu [Member]
Capital contribution agreement		247,000,000us-gaap_OtherCommitment / dei_LegalEntityAxis = cmcsa_HuluMember
Hulu [Member] | NBCUniversal Media, LLC [Member]
Capital contribution agreement		$ 247,000,000us-gaap_OtherCommitment / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / dei_LegalEntityAxis = cmcsa_HuluMember

X

- Definition

This item represents disclosure of the total amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element includes distributions that constitute a return of investment, which are classified as investing activities as well as dividends or distributions classified as operating activities.

+ References

No definition available.

+ Details

Name:	cmcsa_TotalDistributionFromInvestee
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 35

-Paragraph 32

-URI http://asc.fasb.org/extlink&oid=7658923&loc=d3e32787-111569

+ Details

Name:	us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Cash dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporations that are returns of capital. Excludes dividends or distributions from equity method investments classified as operating activities.

+ References

No definition available.

+ Details

Name:	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	cmcsa_LegalEntity1Axis=cmcsa_NBCUniversalMediaLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=cmcsa_HuluMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=cmcsa_WeatherChannelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: