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Debt - Interest Cost Recognized (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Interest cost recognized for the period relating to the:      
Contractual interest coupon rate $ 11 $ 11 $ 11
Amortization of the discount 32 30 29
Total interest cost recognized $ 43 $ 41 $ 40