v2.4.1.9

Investments (Investment Income (Loss), Net) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Gains on sales and exchanges of investments, net	$ 0us-gaap_RealizedInvestmentGainsLosses	$ 83us-gaap_RealizedInvestmentGainsLosses
Investment impairment losses	(15)us-gaap_ImpairmentOfInvestments	(5)us-gaap_ImpairmentOfInvestments
Unrealized gains (losses) on securities underlying prepaid forward sale agreements	42us-gaap_UnrealizedGainLossOnSecurities	(113)us-gaap_UnrealizedGainLossOnSecurities
Mark to market adjustments on derivative component of prepaid forward sale agreements and indexed debt instruments	(38)cmcsa_MarkToMarketAdjustmentOnDerivativeComponentOfPrepaidForwardSaleAgreements	117cmcsa_MarkToMarketAdjustmentOnDerivativeComponentOfPrepaidForwardSaleAgreements
Interest and dividend income	28us-gaap_InvestmentIncomeInterestAndDividend	28us-gaap_InvestmentIncomeInterestAndDividend
Other, net	16cmcsa_OtherInvestmentIncomeLossNet	3cmcsa_OtherInvestmentIncomeLossNet
Investment income (loss), net	33us-gaap_InvestmentIncomeNonoperating	113us-gaap_InvestmentIncomeNonoperating
NBCUniversal Media, LLC [Member]
Investment income (loss), net	$ (2)us-gaap_InvestmentIncomeNonoperating / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	$ 6us-gaap_InvestmentIncomeNonoperating / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember

X

- Definition

Mark To Market Adjustment on Derivative Component Of Prepaid Forward Sale Agreements and indexed debt instruments.

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- Definition

Other Investment Income (Loss), net

+ References

No definition available.

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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560

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- Definition

Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(a),(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a, b

-Article 5

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- Definition

The aggregate amount of income from investments (for example, dividends) not considered a component of the entity's core operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688



Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

-Section 03

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- Definition

The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.3(a))

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 325

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6485959&loc=d3e22529-158502



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 3

-Subparagraph a

-Article 7

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Amount of unrealized gain (loss) recognized in the income statement on unsold other securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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