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Investment Securities - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
Investmentsecurities
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Investment Securities [Line Items]      
Investment securities sold     $ 2,000
Sale of classified investment securities $ 24,796,000 $ 0  
Investment securities carrying value 285,596,000 $ 292,737,000  
Investment securities cost basis 25,757,000    
Gain or loss on sale of investment securities 961,000   $ 0
Residential real estate mortgage-backed securities $ 1,109,966,000    
Life of mortgage-backed securities 5 years 10 months 25 days    
Obligations of U.S. Government Corporations and Agencies [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 69    
Percentage of aggregate depreciation in unrealized losses 1.16%    
Obligations of States and Political Subdivisions [Member]      
Investment Securities [Line Items]      
Debt securities | Investmentsecurities 29    
Percentage of aggregate depreciation in unrealized losses 1.38%    
Marketable Equity Securities [Member]      
Investment Securities [Line Items]      
Percentage of aggregate depreciation in unrealized losses 2.07%