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Investment securities - Additional Information (Details)
$ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Jan. 31, 2017
security
Mar. 31, 2018
USD ($)
security
Mar. 31, 2017
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2016
Dec. 31, 2017
USD ($)
Available-for-sale and Held-to-maturity securities            
Number of investment securities held   198        
Other-than-temporary impairments recognized | $   $ 0        
Collateral Pledged            
Available-for-sale and Held-to-maturity securities            
Carrying value of securities pledged | $   $ 156,522       $ 163,936
State and Municipals, Tax-exempt            
Available-for-sale and Held-to-maturity securities            
Number of investment securities held   114        
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   36        
Number of securities in continuous unrealized loss positions 12 months or longer   20        
Other-than-temporary impairments recognized | $     $ 0 $ 0    
Mortgage-backed Securities, U.S. Government-sponsored enterprises            
Available-for-sale and Held-to-maturity securities            
Number of investment securities held   40        
Number of securities in continuous unrealized loss positions 12 months or longer   16        
U.S. Treasuries            
Available-for-sale and Held-to-maturity securities            
Number of investment securities held 8          
Number of securities in continuous unrealized loss positions 12 months or longer   1