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Leases - Rental Expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Leases [Abstract]    
Minimum rentals $ 2,528 $ 2,455
Contingent rentals 28 29
Rental expense 2,556 2,484
Less: sublease income (21) (24)
Total rental expense, net $ 2,535 $ 2,460