v2.4.1.9

Long-Term Debt (Debt Repayments) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Repurchases and repayments of debt | $ 3,175,000,000 us-gaap_RepaymentsOfOtherDebt | $ 2,444,000,000 us-gaap_RepaymentsOfOtherDebt | $ 2,881,000,000 us-gaap_RepaymentsOfOtherDebt
NBCUniversal Media, LLC [Member] |  |  |
Repurchases and repayments of debt | 906,000,000 us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | 92,000,000 us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | 413,000,000 us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember
NBCUniversal Enterprise Credit Facility [Member] |  |  |
Repurchases and repayments of debt | 1,250,000,000 us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_NbcuniversalEnterpriseCreditFacilityMember |  |
Senior 5.3% Notes Due 2014 [Member] |  |  |
Repurchases and repayments of debt | 1,000,000,000 us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_Senior53NotesDue2014Member |  |
Interest rate | 5.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior53NotesDue2014Member |  |
Maturity date | 2014 |  |
Senior 2.1% Notes Due 2014 [Member] |  |  |
Repurchases and repayments of debt | 900,000,000 us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_Senior21NotesDue2014Member |  |
Interest rate | 2.10%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior21NotesDue2014Member |  |
Maturity date | 2014 |  |
Senior 2.1% Notes Due 2014 [Member] | NBCUniversal Media, LLC [Member] |  |  |
Repurchases and repayments of debt | 900,000,000 us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_Senior21NotesDue2014Member / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Interest rate | 2.10%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = cmcsa_Senior21NotesDue2014Member / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |  |
Maturity date | 2014 |  |
Other Long-Term Debt [Member] |  |  |
Repurchases and repayments of debt | $ 25,000,000 us-gaap_RepaymentsOfOtherDebt / us-gaap_DebtInstrumentAxis = cmcsa_OtherLongTermDebtMember |  |

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_NbcuniversalEnterpriseCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=cmcsa_OtherLongTermDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=cmcsa_Senior21NotesDue2014Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=cmcsa_Senior53NotesDue2014Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |