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Employee benefit plans - Salaries and Employee Benefits Expense - SERP and Employee Pension Plan (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jun. 22, 2008
Supplemental Executive Retirement Plans ("SERP")        
Defined Benefit Plan Disclosure        
Maximum annual benefit in excess of federal limits (as a percent) 6.00%      
Employee benefit plan liability $ 142 $ 133    
Employee benefit plan expense 9 17 $ 20  
Defined benefit plan accrued liabilities 2,351 2,077    
Salaries and employee benefits expense $ 461 360 $ 335  
Pension Benefits        
Defined Benefit Plan Disclosure        
Retirement age period for fixed benefits payable 65 years      
Benefits accrued under employees' pension plan       $ 0
Increase (decrease) in accumulated benefit obligation $ 2,159 $ 1,221