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Fair value of financial instruments and investments - Convertible Senior Notes (Details) - 3.00% Convertible senior notes due 2022 - Convertible debt - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Aug. 31, 2015
Financial assets and liabilities measured at fair value on recurring basis      
Principal $ 150,000,000 $ 150,000,000 $ 150,000,000
Interest rate     3.00%
Debt instrument $ 115,700,000 $ 85,200,000