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Summary of significant accounting policies - Net Sales in United States and Outside of United States (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]                                
Net sales $ 92,671 $ 88,562 $ 72,350 $ 64,527 $ 74,712 $ 80,760 $ 67,615 $ 59,764 $ 66,141 $ 78,571 $ 63,889 $ 59,055 $ 65,920 $ 318,110 $ 282,851 $ 267,435
United States                                
Revenues from External Customers and Long-Lived Assets [Line Items]                                
Net sales                 59,797         284,203 255,284 241,159
International                                
Revenues from External Customers and Long-Lived Assets [Line Items]                                
Net sales                 $ 6,344         $ 33,907 $ 27,567 $ 26,276