The aggregate principal amount of the debt being exchanged in a debt exchange transaction.

No definition available.

Name:	cmcsa_DebtExchangeAmount
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

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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

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

- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.9)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

Long-Term Debt (Senior Notes Exchange) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest expense		$ 3,086	$ 2,942	$ 2,702
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Comcast and NBCUniversal Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 5,500
Aggregate principal amount of certain series of outstanding notes	3,932
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 6.95% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 1,212
Interest rate	6.95%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 6.45% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 946
Interest rate	6.45%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 6.40% Notes Due 2040 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 518
Interest rate	6.40%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| NBCUniversal Senior 6.40% Notes Due 2040 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 442
Interest rate	6.40%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 6.40% Notes Due 2038 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 428
Interest rate	6.40%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 6.55% Notes Due 2039 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 386
Interest rate	6.55%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 3.97% Notes Due 2047 [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 2,000
Interest rate	3.97%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 4.00% Notes Due 2049 [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 2,000
Interest rate	4.00%
Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 4.05% Notes Dues 2052 [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 1,500
Interest rate	4.05%
NBCUniversal Media LLC [Member]
Debt Instrument [Line Items]
Interest expense		$ 727	$ 595	$ 495
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| Comcast and NBCUniversal Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense	$ 157
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| 4.00% Notes Due 2049 To Comcast [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 610
Interest rate	4.00%
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| Senior Notes [Member] \| Comcast and NBCUniversal Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 3,900
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 6.40% Notes Due 2040 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of certain series of outstanding notes	$ 442
Interest rate	6.40%
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 3.97% Notes Due 2047 [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 2,000
Interest rate	3.97%
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 4.00% Notes Due 2049 [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 2,000
Interest rate	4.00%
NBCUniversal Media LLC [Member] \| Senior Notes Exchange [Member] \| Senior Notes [Member] \| Senior 4.05% Notes Due 2052 [Member]
Debt Instrument [Line Items]
Aggregate principal amount issued	$ 1,500
Interest rate	4.05%