v2.4.1.9

Long-term Debt (Debt Repayments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015
Repurchases and repayments of debt	$ 909us-gaap_RepaymentsOfOtherDebt	$ 2,260us-gaap_RepaymentsOfOtherDebt
NBCUniversal Media, LLC [Member]
Repurchases and repayments of debt	1us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember	1us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Senior 6.50% Notes Due 2015 [Member]
Repurchases and repayments of debt	900us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_Senior650NotesDue2015Member
Interest rate	6.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior650NotesDue2015Member
Maturity date	2015
Senior 3.65% Notes Due 2015 [Member]
Repurchases and repayments of debt			1,000us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_Senior365NotesDue2015Member
Interest rate			3.65%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior365NotesDue2015Member
Maturity date			2015
Senior 3.65% Notes Due 2015 [Member] | NBCUniversal Media, LLC [Member]
Repurchases and repayments of debt			$ 1,000us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_Senior365NotesDue2015Member / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Interest rate			3.65%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior365NotesDue2015Member / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
Maturity date			2015

X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

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

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

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

+ Details

Name:	us-gaap_DebtInstrumentMaturityDateDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The cash outflow for the payment of other borrowing not otherwise defined in the taxonomy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

Name:	us-gaap_RepaymentsOfOtherDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

Name:	dei_LegalEntityAxis=cmcsa_NBCUniversalMediaLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior365NotesDue2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior650NotesDue2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: