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Debt - Schedule of Notes (Details) - Convertible Senior Notes - 4% Convertible Senior Dotes due 2024 - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Principal amount (face value) $ 207 $ 207
Unamortized issuance cost (4) (5)
Unamortized debt discount (40) (42)
4% Convertible Senior Notes due 2024 (Carrying value) $ 163 $ 160