v2.4.1.9

Long-Term Debt (Debt Borrowings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Senior 3.60% Notes Due 2024 [Member]
Principal amount	$ 1,200us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = cmcsa_Senior360NotesDue2024Member
Interest rate	3.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior360NotesDue2024Member
Maturity date	2024
Senior 4.75% Notes Due 2044 [Member]
Principal amount	1,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = cmcsa_Senior475NotesDue2044Member
Interest rate	4.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior475NotesDue2044Member
Maturity date	2044
Senior 4.200% Notes Due 2034 [Member]
Principal amount	1,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = cmcsa_Senior4200NotesDue2034Member
Interest rate	4.20%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior4200NotesDue2034Member
Maturity date	2034
Senior 3.375% Notes Due 2025 [Member]
Principal amount	1,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = cmcsa_Senior3375NotesDue2025Member
Interest rate	3.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior3375NotesDue2025Member
Maturity date	2025
Current Year Borrowings [Member]
Principal amount	$ 4,200us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = cmcsa_CurrentYearBorrowingsMember

X

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

+ Details

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

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

- Details

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

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior3375NotesDue2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior360NotesDue2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior4200NotesDue2034Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior475NotesDue2044Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: