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Significant accounting policies - Use of estimates (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Revenue recognition      
Number of bi-annual preventative maintenance visits included in implied warranty and extended service contracts 2    
Term of average selling prices used to determine estimate of current sales price 12 months    
Instrument and accessories      
Revenue recognition      
Standard warranty period 1 year    
Implied warranty period 1 year    
Collaboration and license revenue      
Revenue recognition      
Related party revenue $ 2,150 $ 1,074 $ 172