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Investment securities - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2014
Security
Sep. 30, 2013
Dec. 31, 2013
Security
Schedule of Available-for-sale Securities [Line Items]      
Carrying value of securities pledged $ 215,113   $ 202,407
Number of investment securities held 114   153
Other-than-temporary impairments recognized $ 0 $ 0  
State and Municipals, Tax-Exempt [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Number of investment securities held 28   79
Number of securities in continuous unrealized loss positions 12 months or longer 9    
State and Municipals, Taxable [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Number of investment securities held 2   16
U.S. Government-Sponsored Enterprises State and Municipals [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Maximum percentage of stockholders' equity exceeded for securities of any individual issuer 10.00%   10.00%
Number of investment securities held 26   39
Number of securities in continuous unrealized loss positions 12 months or longer 7   1
Mortgage-backed Securities, Issued by U.S. Government Agencies [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Number of investment securities held 33   16
Corporate Debt Securities [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Number of investment securities held     1
U.S. Treasury Securities [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Number of investment securities held 25    
Common Stock [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Number of investment securities held     2