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Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure      
Interest cost $ 1 $ 1 $ 1
Expected return on plan assets (7) (6) (6)
Prior service credit amortization 1 1 1
Actuarial loss amortization 0 (1) (1)
Net periodic benefit cost $ (5) $ (5) $ (5)