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Investment securities - Additional Information (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
security
Mar. 31, 2015
USD ($)
Dec. 31, 2015
USD ($)
security
Available-for-sale and Held-to-maturity securities      
Carrying value of securities pledged | $ $ 163,514   $ 180,478
Number of investment securities held 60   88
State and Municipals, Tax-exempt      
Available-for-sale and Held-to-maturity securities      
Number of investment securities held 31   38
Number of securities in continuous unrealized loss positions 12 months or longer     4
State and Municipals, Taxable      
Available-for-sale and Held-to-maturity securities      
Number of investment securities held 1   1
Number of securities in continuous unrealized loss positions 12 months or longer 1   1
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 3   12
Number of securities in continuous unrealized loss positions 12 months or longer     7
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 25   36
Number of securities in continuous unrealized loss positions 12 months or longer 6    
Mortgage-backed Securities, U.S. Government-sponsored enterprises | State and Municipals, Tax-exempt      
Available-for-sale and Held-to-maturity securities      
Number of securities in continuous unrealized loss positions 12 months or longer 3