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Leases (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Leases      
Rental expense $ 51 $ 53 $ 52
Minimum lease payments due on leases      
2018 45    
2019 40    
2020 31    
2021 25    
2022 20    
Balance thereafter $ 36