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Stock-based compensation - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Minimum      
Stock-based compensation      
Vested and expected to vest range based on shareholder return 50.00%    
Maximum      
Stock-based compensation      
Vested and expected to vest range based on shareholder return 150.00%    
Restricted Stock Units (RSUs)      
Stock-based compensation      
Stock-Based Compensation, vesting rights Unless otherwise stated in an agreement, the compensation cost associated with the shares of restricted stock is determined based on a two-prong vesting schedule. The first part is vested ratably over five years commencing at the date of grant (“the graduated vesting portion”) and the second part is vested at termination of employment after attainment of 55 years of age and 10 years of service (“the retirement vesting portion”). The graduated vesting portion is accelerated at termination of employment after attaining 55 years of age and 10 years of service. The vesting schedule for restricted shares granted on or after 2014 is as follows, the graduated vesting portion is vested ratably over four years commencing at the date of the grant and the retirement vesting portion is vested at termination of employment after attainment of the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service. The graduated vesting portion is accelerated at termination of employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service. Restricted stock granted on or after 2021 will vest ratably in equal annual installments over a period of 4 years or 3 years, depending on the classification of the employee.    
Restricted Stock Units (RSUs) | Executive Officers      
Stock-based compensation      
Stock granted 53,239 87,706  
Restricted Stock Expense $ 3,900 $ 3,600  
Incentive award, tax benefit 500 $ 400  
Fair Market Value of stock vested at vesting date 4,300    
Windfall Net Of Shortfall 1,700    
Restricted Stock Units (RSUs) | Executive Officers | Share Vesting On Grant Date      
Stock-based compensation      
Fair Market Value of stock vested at vesting date $ 3,200    
Restricted Stock Units (RSUs) | Directors      
Stock-based compensation      
Stock granted 524 298 43,866
Restricted Stock Expense $ 29 $ 18  
Incentive award, tax benefit 5 $ 2  
Fair Market Value of stock vested at vesting date 29    
Performance Based Shares | Executive Officers      
Stock-based compensation      
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management $ 9,900    
Stock granted 71,374 64,815  
Restricted Stock Expense $ 4,300 $ 2,500  
Incentive award, tax benefit $ 400 $ 200  
Stock-Based Compensation, vesting rights The number of shares that will ultimately vest ranges from 50% to a 150% of target based on both market (TSR) and performance (EPS, ROA and ROATCE) conditions. The performance shares vest at the end of the three-year performance cycle. If a participant terminates employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service, the performance shares shall continue outstanding and vest at the end of the performance cycle.    
Performance Based Shares | Executive Officers | Weighted Average      
Stock-based compensation      
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management, period of recognition (in years) 2 years 2 months 15 days