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Stock-based compensation - Narrative (Details) - shares
1 Months Ended 3 Months Ended 4 Months Ended
Nov. 20, 2019
Jul. 01, 2019
Nov. 30, 2019
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options granted (in shares)       0 0  
Equity Incentive Plan 2009            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Additional shares authorized (in shares)   1,666,666        
Shares expired (in shares)     5,342,000      
Equity Incentive Plan 2019            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Additional shares authorized (in shares)           483,000
Shares reserved for future issuance (in shares)     5,700,000      
Additional shares reserved for future issuance (up to) (in shares)     5,800,000 5,800,000   5,800,000
Termination period from date of adoption 10 years