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Investment Securities - Additional Information (Detail)
$ in Millions
Jun. 30, 2015
USD ($)
Securities
Mortgages
Dec. 31, 2014
USD ($)
Jun. 30, 2014
USD ($)
Schedule of Available-for-Sale Securities [Line Items]      
Securities, carrying value | $ $ 323.9 $ 286.6 $ 228.3
Debt Securities [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Number of securities, Total 443    
Number of securities in unrealized loss position 163    
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Number of securities in unrealized loss position | Mortgages 114    
State, County and Municipal Securities [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Number of securities in unrealized loss position 40    
U.S. Government Sponsored Agencies [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Number of securities in unrealized loss position 3    
Corporate Debt Securities [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Number of securities in unrealized loss position 6