Amount of debt discount (premium) and other borrowings.

No definition available.

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

- 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 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: 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=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: 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

- 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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior3.15NotesDue2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_FloatingRateNotesDue2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior3.0NotesDue2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior3.3NotesDue2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior4.45NotesDue2047Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmcsa_Senior4.00NotesDue2047Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt (Debt Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Proceeds from borrowings	$ 11,466	$ 9,231	$ 5,486
Universal Studios Japan Term Loans [Member]
Debt Instrument [Line Items]
Debt face amount	3,948
Other Borrowings [Member]
Debt Instrument [Line Items]
Other borrowings	13
Senior Notes [Member] \| Senior 3.15% Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt face amount	$ 1,650
Interest rate	3.15%
Senior Notes [Member] \| Floating Rate Notes Due 2021 [Member]
Debt Instrument [Line Items]
Debt face amount	$ 1,500
Senior Notes [Member] \| Senior 3.00% Notes Due 2024 [Member]
Debt Instrument [Line Items]
Debt face amount	$ 1,250
Interest rate	3.00%
Senior Notes [Member] \| Senior 3.30% Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt face amount	$ 1,250
Interest rate	3.30%
Senior Notes [Member] \| Senior 4.45% Notes Due 2047 [Member]
Debt Instrument [Line Items]
Debt face amount	$ 1,005
Interest rate	4.45%
Senior Notes [Member] \| Senior 4.00% Notes Due 2047 [Member]
Debt Instrument [Line Items]
Debt face amount	$ 850
Interest rate	4.00%