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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, 2017
Dec. 31, 2016
Dec. 31, 2015
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 $ 96 $ 76    
Employee benefit plan expense 20 13 $ 15  
Defined benefit plan accrued liabilities 1,594 1,416    
Salaries and employee benefits expense $ 314 254 $ 157  
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 $ 395 $ (256)