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Summary of significant accounting policies - Reconciliation of Sales Allowances (Details) - Sales Allowances - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2018
Sales Allowances [Roll Forward]        
Beginning balance $ 7,774 $ 7,613 $ 6,545 $ 8,458
Charges 6,787 41,027 29,576 26,971
Deductions (8,016) (36,727) (28,508) (27,655)
Ending balance $ 6,545 $ 11,913 $ 7,613 $ 7,774