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Stock-based compensation - Weighted Average Basis Assumptions (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Stock-based compensation        
Risk-free interest rate 0.96% 0.34% 0.90% 0.36%
Expected term (in years) 5 years 9 months 18 days 6 years 6 years 6 years
Expected volatility 43.70% 90.40% 51.50% 90.30%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%