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Significant accounting policies - Stock-based compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2019
New Accounting Pronouncement, Early Adoption        
Expected dividend yield 0.00% 0.00% 0.00%  
Expected term (in years) 6 years 5 years 10 months 24 days 6 years  
Weighted-average fair value of options granted $ 9.09 $ 7.19 $ 4.52  
Accumulated deficit $ (216,244) $ (175,888)   $ (175,448)
Minimum        
New Accounting Pronouncement, Early Adoption        
Risk-free interest rate 1.40% 2.60% 1.80%  
Expected volatility 33.50% 32.40% 46.00%  
Maximum        
New Accounting Pronouncement, Early Adoption        
Risk-free interest rate 2.60% 3.00% 2.10%  
Expected volatility 39.70% 36.80% 52.00%