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Commitments and Contingencies (Rental expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Commitment And Contingencies [Line Items]    
Rental expense $ 779 $ 839
NBCUniversal Media LLC [Member]    
Commitment And Contingencies [Line Items]    
Rental expense $ 286 $ 274