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Debt - Interest Expense (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Debt Disclosure [Abstract]        
Contractual interest at coupon rate $ 1 $ 3 $ 5 $ 9
Amortization of the discount 5 8 18 24
Interest expense, debt $ 6 $ 11 $ 23 $ 33