v2.4.1.9

Fair Value Disclosure Rollforward (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Nonrecurring fair value adjustments | $ 26 us-gaap_AssetImpairmentCharges | $ 167 us-gaap_AssetImpairmentCharges | $ 161 us-gaap_AssetImpairmentCharges
NBCUniversal Media, LLC [Member] |  |  |
Nonrecurring fair value adjustments | 26 us-gaap_AssetImpairmentCharges / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | 167 us-gaap_AssetImpairmentCharges / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | 161 us-gaap_AssetImpairmentCharges / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Contractual Obligation [Member] |  |  |
Beginning balance | 747 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember |  |
Fair value adjustments | 208 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember |  |
Payments | (72)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember |  |
Ending balance | 883 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember |  |
Contractual Obligation [Member] | NBCUniversal Media, LLC [Member] |  |  |
Beginning balance | 747 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Fair value adjustments | 208 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Payments | (72)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Ending balance | 883 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ObligationsMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Contingent Consideration [Member] |  |  |
Beginning balance | 684 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember |  |
Fair value adjustments | 31 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember |  |
Payments | (71)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember |  |
Ending balance | $ 644 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = cmcsa_ContingentConsiderationMember |  |
Maximum [Member] |  |  |
Discount rate used to determine fair value | 13.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MaximumMember |  |
Maximum [Member] | NBCUniversal Media, LLC [Member] |  |  |
Discount rate used to determine fair value | 13.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MaximumMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Minimum [Member] |  |  |
Discount rate used to determine fair value | 5.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MinimumMember |  |
Minimum [Member] | NBCUniversal Media, LLC [Member] |  |  |
Discount rate used to determine fair value | 12.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = us-gaap_MinimumMember / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |

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- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + 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 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228 + Details
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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of increase (decrease) of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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