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Debt - Interest Expense (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Debt Disclosure [Abstract]        
Contractual interest at coupon rate $ 2 $ 3 $ 4 $ 6
Amortization of the discount 6 8 13 16
Interest expense, debt $ 8 $ 11 $ 17 $ 22