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Employee benefit plans - Fair Value Assumptions for Stock Options (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Equity Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected Term 6.1 6.1 5.4-6.1
Dividend yield 0.00% 0.00% 0.00%
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]      
Expected Term 6.1 6.1 5.4-6.1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expected Term 6.1 6.1 5.4-6.1
Employee Stock Purchase Plan Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected Term 0.5 0.5 0.5
Dividend yield 0.00% 0.00% 0.00%
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]      
Expected Term 0.5 0.5 0.5
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expected Term 0.5 0.5 0.5