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Debt - Components of convertible notes (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Convertible Notes    
Convertible senior notes noncurrent $ 17,647 $ 17,434 [1]
Carrying Amount    
Convertible Notes    
Convertible senior notes 17,647 17,434
Convertible senior notes noncurrent $ 17,647 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 $ 17,647 $ 17,434
[1] The condensed consolidated balance sheet at December 31, 2018 has been derived from the audited financial statements at that date.