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Investment Securities - Additional Information (Detail)
3 Months Ended
Mar. 31, 2016
USD ($)
Investmentsecurities
Mar. 31, 2015
USD ($)
Dec. 31, 2015
USD ($)
Investment Securities [Line Items]      
Investment securities sold | $ $ 0 $ 0  
Investment securities carrying value | $ 288,887,000   $ 297,547,000
Mortgage-backed securities | $ $ 1,044,980,000    
Life of mortgage-backed securities 4 years 9 months 18 days    
Obligations of U.S. Government Corporations and Agencies [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 5    
Percentage of aggregate depreciation in unrealized losses 0.17%    
Obligations of States and Political Subdivisions [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 15    
Percentage of aggregate depreciation in unrealized losses 0.93%    
Marketable Equity Securities [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 0