Contribution from member including, but not limited to, a business, investment or asset.

No definition available.

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

- Definition

[Line Items] for Schedule of Fair Value Method Investments [Table]

+ References

No definition available.

+ Details

Name:	cmcsa_ScheduleofFairValueMethodInvestmentsLineItems
Namespace Prefix:	cmcsa_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- Definition

Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

+ Details

Name:	us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash outflow for purchase of marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26853-111562

+ Details

Name:	us-gaap_PaymentsToAcquireMarketableSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=cmcsa_SnapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments (Marketable Equity Securities) (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Marketable Equity Securities [Line Items]
Marketable equity securities			$ 341	$ 433
NBCUniversal Media LLC [Member]
Schedule of Marketable Equity Securities [Line Items]
Contribution from member				662	$ 3,655
Marketable equity securities			162	430
Snap [Member]
Schedule of Marketable Equity Securities [Line Items]
Payments to acquire marketable securities		$ 500
Marketable equity securities			162	430
Marketable securities, recognized unrealized gains (losses)			(268)
Snap [Member] \| NBCUniversal Media LLC [Member]
Schedule of Marketable Equity Securities [Line Items]
Contribution from member	$ 662
Marketable equity securities			162	$ 430
Marketable securities, recognized unrealized gains (losses)			$ (268)