v2.4.1.9

Investments (Investment Income (Loss), Net) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Gains on sales and exchanges of investments, net | $ 192 us-gaap_RealizedInvestmentGainsLosses | $ 484 us-gaap_RealizedInvestmentGainsLosses | $ 30 us-gaap_RealizedInvestmentGainsLosses
Investment impairment losses | (50)us-gaap_ImpairmentOfInvestments | (29)us-gaap_ImpairmentOfInvestments | (24)us-gaap_ImpairmentOfInvestments
Unrealized gains on securities underlying prepaid forward sale agreements | 66 us-gaap_UnrealizedGainLossOnSecurities | 1,601 us-gaap_UnrealizedGainLossOnSecurities | 1,159 us-gaap_UnrealizedGainLossOnSecurities
Mark to market adjustments on derivative component of prepaid forward sale agreements and indexed debt instruments | (56)cmcsa_MarkToMarketAdjustmentOnDerivativeComponentOfPrepaidForwardSaleAgreements | (1,604)cmcsa_MarkToMarketAdjustmentOnDerivativeComponentOfPrepaidForwardSaleAgreements | (1,071)cmcsa_MarkToMarketAdjustmentOnDerivativeComponentOfPrepaidForwardSaleAgreements
Interest and dividend income | 116 us-gaap_InvestmentIncomeInterestAndDividend | 111 us-gaap_InvestmentIncomeInterestAndDividend | 119 us-gaap_InvestmentIncomeInterestAndDividend
Other, net | 28 cmcsa_OtherInvestmentIncomeLossNet | 13 cmcsa_OtherInvestmentIncomeLossNet | 6 cmcsa_OtherInvestmentIncomeLossNet
Investment income (loss), net | 296 us-gaap_InvestmentIncomeNonoperating | 576 us-gaap_InvestmentIncomeNonoperating | 219 us-gaap_InvestmentIncomeNonoperating
NBCUniversal Media, LLC [Member] |  |  |
Investment income (loss), net | $ 27 us-gaap_InvestmentIncomeNonoperating / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | $ 17 us-gaap_InvestmentIncomeNonoperating / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | $ 27 us-gaap_InvestmentIncomeNonoperating / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember

X
- Definition Mark To Market Adjustment on Derivative Component Of Prepaid Forward Sale Agreements and indexed debt instruments. + References No definition available. + Details
Name: | cmcsa_MarkToMarketAdjustmentOnDerivativeComponentOfPrepaidForwardSaleAgreements
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Other Investment Income (Loss), net + References No definition available. + Details
Name: | cmcsa_OtherInvestmentIncomeLossNet
Namespace Prefix: | cmcsa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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 + Details
Name: | us-gaap_ImpairmentOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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 + Details
Name: | us-gaap_InvestmentIncomeInterestAndDividend
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- 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 -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Article 5 + Details
Name: | us-gaap_InvestmentIncomeNonoperating
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- 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 + Details
Name: | us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition 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 + Details
Name: | us-gaap_UnrealizedGainLossOnSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=cmcsa_NBCUniversalMediaLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |