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Investment Securities - Additional Information (Detail)
9 Months Ended
Sep. 30, 2015
USD ($)
Investmentsecurities
Sep. 30, 2014
USD ($)
Dec. 31, 2014
USD ($)
Investment Securities [Line Items]      
Investment securities sold $ 3,000 $ 0  
Investment securities carrying value 310,429,000   $ 143,992,000
Mortgage-backed securities $ 1,006,998,000    
Life of mortgage-backed securities 5 years    
Obligations of U.S. government corporations and agencies [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 17    
Percentage of aggregate depreciation in unrealized losses (0.53%)    
Obligations of states and political subdivisions [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 23    
Percentage of aggregate depreciation in unrealized losses (2.01%)    
Marketable equity securities [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 0