v2.4.1.9
Investments (Cost Method Investments) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Air Touch [Member]    
Fair value of AirTouch preferred stock related to redeemable preferred shares $ 1,700,000,000cmcsa_FairValueOfCostMethodInvestmentRedeemablePreferredShares
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember		 $ 1,700,000,000cmcsa_FairValueOfCostMethodInvestmentRedeemablePreferredShares
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember
Aggregate redemption value 1,750,000,000cmcsa_PreferredStockAggregateRedemptionValue
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember		  
Carrying amount of redeemable preferred stock 1,600,000,000cmcsa_CarryingAmountOfRedeemablePreferredStock
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember		 1,500,000,000cmcsa_CarryingAmountOfRedeemablePreferredStock
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember
Fair value of redeemable preferred stock 1,700,000,000cmcsa_FairValueAmountOfRedeemablePreferredStock
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember		 1,700,000,000cmcsa_FairValueAmountOfRedeemablePreferredStock
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember
Nonredeemable preferred stock 100,000,000cmcsa_NonredeemablePreferredStock
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember		 100,000,000cmcsa_NonredeemablePreferredStock
/ dei_LegalEntityAxis
= cmcsa_AirTouchMember
Preferred Shares Redeemable in April 2020 [Member]    
Aggregate redemption value $ 1,650,000,000cmcsa_PreferredStockAggregateRedemptionValue
/ dei_LegalEntityAxis
= cmcsa_PreferredSharesRedeemableInApril2020Member