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Debt - Components of convertible notes (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Convertible Notes      
Less current portion $ 7,460 $ 7,393 [1]  
Convertible senior notes noncurrent 16,811 16,608 [1]  
Carrying Amount      
Convertible Notes      
Convertible senior notes, total 24,271 24,001  
Less current portion 7,460 7,393  
Convertible senior notes noncurrent $ 16,811 16,608  
Convertible Senior Notes 0.375 Percent Due 2018      
Convertible Notes      
Interest rate of debt (as a percent) 0.375%   0.375%
Convertible Senior Notes 0.375 Percent Due 2018 | Carrying Amount      
Convertible Notes      
Convertible senior notes, total $ 7,460 7,393  
Convertible Senior Notes 1.25 Percent Due 2020      
Convertible Notes      
Interest rate of debt (as a percent) 1.25%   1.25%
Convertible Senior Notes 1.25 Percent Due 2020 | Carrying Amount      
Convertible Notes      
Convertible senior notes, total $ 16,811 $ 16,608  
[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date.