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Fair value of financial instruments and investments - Convertible senior notes (Details) - 1.50% Convertible senior notes due 2026 - USD ($)
Sep. 30, 2023
Dec. 31, 2022
Sep. 30, 2019
Long-term debt      
Interest rate ( as a percent ) 1.50%    
Convertible debt      
Long-term debt      
Principal $ 287,500,000 $ 287,500,000 $ 287,500,000
Interest rate ( as a percent ) 1.50%   1.50%
Convertible debt | Significant other observable inputs (level 2)      
Long-term debt      
Fair value of convertible notes $ 228,800,000 $ 281,700,000