v2.4.1.9

Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair value method	$ 182us-gaap_MarketableSecurities	$ 662us-gaap_MarketableSecurities
Equity method	1,149us-gaap_EquityMethodInvestments	1,019us-gaap_EquityMethodInvestments
Cost method	2,062us-gaap_CostMethodInvestments	2,056us-gaap_CostMethodInvestments
Total Investments	3,393us-gaap_Investments	3,737us-gaap_Investments
Less: Current investments	158us-gaap_ShortTermInvestments	602us-gaap_ShortTermInvestments
Noncurrent investments	3,235us-gaap_LongTermInvestments	3,135us-gaap_LongTermInvestments
NBCUniversal Media, LLC [Member]
Fair value method	10us-gaap_MarketableSecurities / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	10us-gaap_MarketableSecurities / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Equity method	971us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	840us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Cost method	24us-gaap_CostMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	32us-gaap_CostMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Total Investments	1,005us-gaap_Investments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	882us-gaap_Investments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Less: Current investments	52us-gaap_ShortTermInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	0us-gaap_ShortTermInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Noncurrent investments	953us-gaap_LongTermInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember	882us-gaap_LongTermInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember
Air Touch [Member]
Cost method	1,572us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_AirTouchMember	1,568us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_AirTouchMember
Other Investment [Member]
Equity method	534us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember	517us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember
Cost method	490us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember	488us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember
Other Investment [Member] | NBCUniversal Media, LLC [Member]
Equity method	356us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember	338us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_OtherInvestmentMember
The Weather Channel [Member]
Equity method	336us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember	335us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember
The Weather Channel [Member] | NBCUniversal Media, LLC [Member]
Equity method	336us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember	335us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_WeatherChannelMember
Hulu [Member]
Equity method	279us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember	167us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember
Hulu [Member] | NBCUniversal Media, LLC [Member]
Equity method	$ 279us-gaap_EquityMethodInvestments / cmcsa_LegalEntity1Axis = cmcsa_NBCUniversalMediaLLCMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cmcsa_HuluMember	$ 167us-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

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

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-Name Accounting Standards Codification

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-SubTopic 20

-Section 50

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 20

-Section 35

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-URI http://asc.fasb.org/extlink&oid=29638308&loc=d3e40346-111594

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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)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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X

- 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

-Paragraph 1

-Subparagraph (SX 210.7-03.1(h))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph h

-Article 7

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

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

+ 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)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

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-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

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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

-Name Accounting Standards Codification

-Topic 210

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-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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-Name Regulation S-X (SX)

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