Variable interest entity, maximum borrowing capacity under financing agreements.

No definition available.

Name:	cmcsa_VariableInterestEntityMaximumBorrowingCapacity
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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=6879938&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

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5710-111685

+ Details

Name:	us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.

+ References

+ Details

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

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=cmcsa_UniversalBeijingResortMember
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_VariableInterestEntitiesByClassificationOfEntityAxis=cmcsa_UniversalBeijingResortTermLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Significant Transactions (Universal Beijing Resort) (Details) $ in Millions, ¥ in Billions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 CNY (¥)
Variable Interest Entity [Line Items]
Proceeds from borrowings	$ 44,781	$ 11,466	$ 9,231
Universal Beijing Resort [Member]
Variable Interest Entity [Line Items]
Ownership percentage in variable interest entity	30.00%			30.00%
Maximum borrowing capacity of variable interest entity	$ 4,000			¥ 26.6
Consolidated variable interest entity's assets included in condensed consolidated balance sheet	1,500
Consolidated variable interest entity's liabilities included in condensed consolidated balance sheet	1,000
NBCUniversal Media LLC [Member]
Variable Interest Entity [Line Items]
Proceeds from borrowings	$ 692	$ 3,948	$ 0
NBCUniversal Media LLC [Member] \| Universal Beijing Resort [Member]
Variable Interest Entity [Line Items]
Ownership percentage in variable interest entity	30.00%			30.00%
Maximum borrowing capacity of variable interest entity	$ 4,000			¥ 26.6
Consolidated variable interest entity's assets included in condensed consolidated balance sheet	1,500
Consolidated variable interest entity's liabilities included in condensed consolidated balance sheet	1,000
NBCUniversal Media LLC [Member] \| Universal Beijing Resort Term Loans [Member]
Variable Interest Entity [Line Items]
Proceeds from borrowings	569
Universal Beijing Resort Term Loans [Member]
Variable Interest Entity [Line Items]
Other Long-term Debt	$ 569
Universal Beijing Resort Term Loans [Member] \| NBCUniversal Media LLC [Member]
Variable Interest Entity [Line Items]
Other Long-term Debt \| ¥				¥ 4.0