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Commitments and Contingencies (Schedule of Future Minimum Rental Payments) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Leases, Operating [Abstract]      
2018 $ 222    
2019 187    
2020 154    
2021 108    
2022 86    
Thereafter 219    
Total noncancelable operating lease commitments 976    
Rent expense $ 238 $ 234 $ 238