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Nature of the business and basis of presentation - Going Concern (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Nature of the business and basis of presentation                  
Net loss $ (24,987) $ (11,831) $ (22,101) $ (8,465) $ (9,267) $ (8,007) $ (58,919) $ (25,739)  
Accumulated deficit $ (300,507)           $ (300,507)   $ (241,588)