v2.4.0.8

Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Unrecognized tax benefits, interest on income taxes accrued	$ 460	$ 780
Change In Income Tax Expense Related To Effectively Settled Uncertain Tax Positions	$ 724

X

- Definition

Represents the change in income tax expense as a result of a reduction in the accrual for uncertain tax positions and related accrued interest on these tax positions, net of any indemnification.

+ References

No definition available.

+ Details

Name:	cmcsa_ChangeInIncomeTaxExpenseRelatedToEffectivelySettledUncertainTaxPositions
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of interest expense accrued for an underpayment of income taxes.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant