Investments (Equity Method Investments) (Details) - The Weather Channel [Member] $ in Millions	9 Months Ended
Sep. 30, 2015 USD ($)
Goodwill Impairment Loss From Equity Method Investments	$ 252
NBCUniversal Media, LLC [Member]
Goodwill Impairment Loss From Equity Method Investments	$ 252

This item represents the entity's proportionate share for the period of a goodwill impairment charge recorded by its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. The amount is recorded in equity in net income (losses) of investees, net.

No definition available.

Name:	cmcsa_Goodwillimpairmentlossfromequitymethodinvestments
Namespace Prefix:	cmcsa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration