The carrying amount as of the balance sheet date for the net impact of debt issuance costs, premiums and discounts.

No definition available.

Name:	cmcsa_DebtIssuanceCostsPremiumsAndDiscountsNet
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Issuance Costs, Premiums, Discounts And Fair Value Adjustments For Acquisition Accounting And Hedged Positions

+ References

No definition available.

+ Details

Name:	cmcsa_DebtIssuanceCostsPremiumsDiscountsAndFairValueAdjustmentsForAcquisitionAccountingAndHedgedPositions
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of short-term and long-term debt and lease obligation.

+ References

+ Details

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

- Definition

Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

+ References

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

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

- Definition

Amount of long-term debt and lease obligation, classified as noncurrent.

+ References

+ Details

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

- Definition

The carrying amount as of the balance sheet date for the aggregate of other miscellaneous borrowings owed by the reporting entity.

+ References

+ Details

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

- Definition

Amount of long-term debt classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of long-term notes classified as other, payable after one year or the normal operating cycle, if longer.

+ References

+ Details

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

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16(a))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_SeniorNotesWithMaturitiesOf5YearsOrLessatFaceValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_SeniorNotesWithMaturitiesBetween5And10YearsatfacevalueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_SeniorNotesWithMaturitiesGreaterThan10YearsatfacevalueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_A4.00NotesDue2049ToComcastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt (Schedule of Debt Outstanding) (Details) $ in Millions, € in Billions, £ in Billions	Dec. 31, 2019 USD ($)	Dec. 31, 2019 JPY (¥)	Dec. 31, 2019 CNY (¥)	Dec. 31, 2019 EUR (€)	Dec. 31, 2019 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 JPY (¥)	Dec. 31, 2018 CNY (¥)	Dec. 31, 2018 EUR (€)	Dec. 31, 2018 GBP (£)
Debt Instrument [Line Items]
Debt issuance costs, premiums, discounts, fair value adjustments for acquisition accounting and hedged positions, net	$ (1,673)					$ (1,702)
Total debt	102,217					111,743
Less: Current portion	4,452					4,398
Long-term debt	$ 97,765					107,345
Interest rate	3.78%	3.78%	3.78%	3.78%	3.78%
Commercial Paper [Member]
Debt Instrument [Line Items]
Commercial paper	$ 0					675
Interest rate	0.00%	0.00%	0.00%	0.00%	0.00%
Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Revolving credit facility	$ 0					606
Interest rate	0.00%	0.00%	0.00%	0.00%	0.00%
Term Loans [Member]
Debt Instrument [Line Items]
Term loans	$ 8,078					13,268
Interest rate	1.87%	1.87%	1.87%	1.87%	1.87%
Senior notes with maturities of 5 years or less, at face value [Member]
Debt Instrument [Line Items]
Senior notes	$ 26,378					26,331
Interest rate	3.29%	3.29%	3.29%	3.29%	3.29%
Senior notes with maturities between 5 and 10 years, at face value [Member]
Debt Instrument [Line Items]
Senior notes	$ 21,683					26,727
Interest rate	3.74%	3.74%	3.74%	3.74%	3.74%
Senior notes with maturities greater than 10 years, at face value [Member]
Debt Instrument [Line Items]
Senior notes	$ 46,653					45,030
Interest rate	4.54%	4.54%	4.54%	4.54%	4.54%
Other Including Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Other, including capital lease obligations	$ 1,098					808
Foreign Currency Denominated Borrowings [Member]
Debt Instrument [Line Items]
Aggregate principal amount		¥ 267,000,000,000	¥ 9,000,000,000	€ 4.9	£ 4.9		¥ 390,000,000,000	¥ 4,000,000,000	€ 4.9	£ 7.3
NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Debt issuance costs, premiums and discounts, net	(33)					(36)
Total debt	10,010					12,882
Less: Current portion	301					151
Long-term debt	$ 9,709					12,731
Interest rate	3.59%	3.59%	3.59%	3.59%	3.59%
NBCUniversal Media LLC [Member] \| Term Loans [Member]
Debt Instrument [Line Items]
Term loans	$ 3,740					4,122
Interest rate	2.04%	2.04%	2.04%	2.04%	2.04%
NBCUniversal Media LLC [Member] \| Senior notes with maturities of 5 years or less, at face value [Member]
Debt Instrument [Line Items]
Senior notes	$ 3,000					5,000
Interest rate	3.88%	3.88%	3.88%	3.88%	3.88%
NBCUniversal Media LLC [Member] \| Senior notes with maturities between 5 and 10 years, at face value [Member]
Debt Instrument [Line Items]
Senior notes	$ 0					0
Interest rate	0.00%	0.00%	0.00%	0.00%	0.00%
NBCUniversal Media LLC [Member] \| Senior notes with maturities greater than 10 years, at face value [Member]
Debt Instrument [Line Items]
Senior notes	$ 2,759					2,759
Interest rate	5.50%	5.50%	5.50%	5.50%	5.50%
NBCUniversal Media LLC [Member] \| 4.00% Notes Due 2049 To Comcast [Member]
Debt Instrument [Line Items]
Notes due 2049 to Comcast	$ 0					610
Interest rate	0.00%	0.00%	0.00%	0.00%	0.00%
NBCUniversal Media LLC [Member] \| Other Including Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Other, including capital lease obligations	$ 544					$ 427
NBCUniversal Media LLC [Member] \| Foreign Currency Denominated Borrowings [Member]
Debt Instrument [Line Items]
Aggregate principal amount		¥ 284,000,000,000	¥ 9,000,000,000				¥ 400,000,000,000	¥ 4,000,000,000