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Commitments and Contingencies, Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Leases, Rent Expense      
Rent expense $ 166 $ 161 $ 153
Loss Contingency, Estimate      
Accrual for legal proceedings and unasserted claims $ 1    
Maximum      
Loss Contingency, Estimate      
Accrual for legal proceedings and unasserted claims   $ 1