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Summary of significant accounting policies - Product Warranties (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Summary of significant accounting policies        
Balance, beginning of the period $ 612 $ 757 $ 637 $ 848
Warranty repairs 0 (3) (25) (94)
Balance, end of the year $ 612 $ 754 $ 612 $ 754
Warranty duration     1 year