v2.4.1.9

Investments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair value method	$ 662,000,000us-gaap_MarketableSecurities	$ 4,345,000,000us-gaap_MarketableSecurities
Equity method	1,019,000,000us-gaap_EquityMethodInvestments	989,000,000us-gaap_EquityMethodInvestments
Cost method	2,056,000,000us-gaap_CostMethodInvestments	2,009,000,000us-gaap_CostMethodInvestments
Total Investments	3,737,000,000us-gaap_Investments	7,343,000,000us-gaap_Investments
Less: Current investments	602,000,000us-gaap_ShortTermInvestments	3,573,000,000us-gaap_ShortTermInvestments
Noncurrent investments	3,135,000,000us-gaap_LongTermInvestments	3,770,000,000us-gaap_LongTermInvestments
NBCUniversal Media, LLC [Member]
Fair value method	10,000,000us-gaap_MarketableSecurities / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	11,000,000us-gaap_MarketableSecurities / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Equity method	840,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	852,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Cost method	32,000,000us-gaap_CostMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	21,000,000us-gaap_CostMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Total Investments	882,000,000us-gaap_Investments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	884,000,000us-gaap_Investments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Other Equity Method Investments [Member]
Equity method	517,000,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherEquityMethodInvestmentsMember	469,000,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherEquityMethodInvestmentsMember
Other Equity Method Investments [Member] | NBCUniversal Media, LLC [Member]
Equity method	338,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherEquityMethodInvestmentsMember	332,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherEquityMethodInvestmentsMember
Air Touch [Member]
Cost method	1,568,000,000us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_AirTouchMember	1,553,000,000us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_AirTouchMember
Other Investment [Member]
Cost method	488,000,000us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember	456,000,000us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember
The Weather Channel [Member]
Equity method	335,000,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember	333,000,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember
The Weather Channel [Member] | NBCUniversal Media, LLC [Member]
Equity method	335,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember	333,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember
Hulu [Member]
Equity method	167,000,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember	187,000,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember
Hulu [Member] | NBCUniversal Media, LLC [Member]
Equity method	$ 167,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember	$ 187,000,000us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember

X

- Definition

Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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X

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

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- Definition

Sum of the carrying amounts as of the balance sheet date of all investments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

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- Definition

The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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X

- Definition

Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

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- Definition

Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

+ References

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

-Publisher FASB

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