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Investment securities - Additional Information (Details)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
security
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Available-for-sale and Held-to-maturity securities          
Number of investment securities held     204    
Other-than-temporary impairments recognized | $     $ 0    
Collateral Pledged          
Available-for-sale and Held-to-maturity securities          
Carrying value of securities pledged | $ $ 153,587   $ 153,587   $ 163,936
State and Municipals, Tax-exempt          
Available-for-sale and Held-to-maturity securities          
Number of investment securities held     112    
Number of securities in continuous unrealized loss positions 12 months or longer     8    
Credit Concentration Risk | Total Stockholders' Equity | US Government Agency and Sponsored Enterprises | Minimum          
Available-for-sale and Held-to-maturity securities          
Maximum percentage of stockholders' equity exceeded for securities of any individual issuer     10.00%   10.00%
U.S. Government-sponsored enterprises state and municipals          
Available-for-sale and Held-to-maturity securities          
Number of investment securities held     38    
Number of securities in continuous unrealized loss positions 12 months or longer     20    
Other-than-temporary impairments recognized | $ $ 0 $ 0 $ 0 $ 0  
Mortgage-backed Securities, U.S. Government-sponsored enterprises          
Available-for-sale and Held-to-maturity securities          
Number of investment securities held     45    
Number of securities in continuous unrealized loss positions 12 months or longer     19    
U.S. Treasuries          
Available-for-sale and Held-to-maturity securities          
Number of investment securities held     9    
Number of securities in continuous unrealized loss positions 12 months or longer     1