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Stock-based compensation - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Service-based Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized stock-based compensation cost $ 7,300,000    
Unrecognized stock-based compensation cost, expected weighted-average period of recognition 2 years 4 months 24 days    
Maximum contractual life 10 years    
Expected dividend yield 0.00% 0.00% 0.00%
Performance-based and Market-based Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized stock-based compensation cost $ 0    
RSA      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unvested shares outstanding (in shares) 885,520    
RSAs and RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized stock-based compensation cost, expected weighted-average period of recognition 2 years 8 months 12 days    
Unrecognized stock-based compensation cost $ 31,700,000    
Unvested shares outstanding (in shares) 2,036,124 1,248,824  
2016 Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized (in shares) 9,366,718    
Shares available for grant (in shares) 4,608,064    
2014 Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options and awards outstanding (in shares) 2,063,122