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Off-balance sheet financial instruments - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Off-balance sheet financial instruments.    
Allowance recorded under off balance sheet credit losses during the period $ 61 $ 58
Expiration period of standby letters 12 months  
Amount of standby letters of credit $ 14,049 $ 27,072