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Employee benefit plans - Fair Value Assumptions for Restricted Stock Units and ESPP (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employee Stock Purchase Plan Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.00% 0.00% 0.00%