v2.4.0.8

Investments (Investment Income (Loss), Net) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jan. 28, 2011 NBCUniversal Media, LLC [Member]	Dec. 31, 2011 NBCUniversal Media, LLC [Member]	Dec. 31, 2013 NBCUniversal Media, LLC [Member]	Dec. 31, 2012 NBCUniversal Media, LLC [Member]
Gains on sales and exchanges of investments, net	$ 484	$ 30	$ 41
Investment impairment losses	(29)	(24)	(5)
Unrealized gains (losses) on securities underlying prepaid forward sale agreements	1,601	1,159	192
Mark to market adjustments on derivative component of prepaid forward sale agreements and indexed debt instruments	(1,604)	(1,071)	(119)
Interest and dividend income	111	119	110
Other, net	13	6	(60)
Investment income (loss), net	$ 576	$ 219	$ 159	$ 4	$ 19	$ 17	$ 27

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