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Stock-based compensation - Additional Information (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2020
Dec. 31, 2018
Dec. 31, 2017
Service-based Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unrecognized stock-based compensation cost   $ 3,300,000    
Unrecognized stock-based compensation cost, expected weighted-average period of recognition   2 years 2 months 1 day    
Maximum contractual life   10 years    
Expected dividend yield 0.00% 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)   1,128,789    
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 4 months 24 days    
Unvested shares outstanding (in shares) 2,786,398 2,311,768 2,036,124  
Unrecognized stock-based compensation cost   $ 25,000,000    
2016 Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized (in shares)   13,457,391    
Shares available for grant (in shares)   7,643,774    
2014 Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of options and awards outstanding (in shares)   1,471,032