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Employee benefit plans - Summary of Warrant Activity (Details) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Number of warrants        
Granted (in shares)     0  
Warrants        
Number of warrants        
Beginning balance, Outstanding (in shares) 261,681 261,681 261,681 261,681
Granted (in shares)     0 0
Exercised (in shares)     0 0
Forfeited (in shares)     0 0
Ending balance, Outstanding (in shares)     261,681 261,681
Weighted-average exercise price        
Weighted-average exercise price, beginning balance (in dollars per share) $ 3.00 $ 3.00 $ 3.00 $ 3.00
Granted (in dollars per share) 0 0
Exercised (in dollars per share) 3.00 0
Forfeited (in dollars per share) 0 0
Weighted-average exercise price, ending balance (in dollars per share) $ 0 $ 3.00 $ 3.00 $ 3.00