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Debt - Components of convertible notes (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Convertible Notes    
Convertible senior notes noncurrent $ 18,080 $ 17,434
Carrying Amount    
Convertible Notes    
Convertible senior notes 18,080 17,434
Convertible senior notes noncurrent $ 18,080 17,434
Convertible Senior Notes 1.25 Percent Due 2020    
Convertible Notes    
Interest rate of debt (as a percent) 1.25%  
Convertible Senior Notes 1.25 Percent Due 2020 | Carrying Amount    
Convertible Notes    
Convertible senior notes $ 18,080 $ 17,434