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 | $ 909 us-gaap_RepaymentsOfOtherDebt | $ 2,260 us-gaap_RepaymentsOfOtherDebt |
NBCUniversal Media, LLC [Member] |  |  |
Repurchases and repayments of debt | 1 us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember | 1 us-gaap_RepaymentsOfOtherDebt / dei_LegalEntityAxis = cmcsa_NBCUniversalMediaLLCMember |
Senior 6.50% Notes Due 2015 [Member] |  |  |
Repurchases and repayments of debt | 900 us-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 |  |  | 1,000 us-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 |  |  | $ 1,000 us-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
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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
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Namespace Prefix: | us-gaap_
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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
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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
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |