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Securities Available for Sale - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Dec. 31, 2015
USD ($)
Schedule of Available-for-sale Securities [Line Items]      
Securities available for sale at carrying value $ 939,400,000 $ 720,400,000  
Amortized cost 1,042,285,000 1,041,247,000  
Securities available for sale, at fair value 1,037,812,000 1,040,386,000  
Proceeds from the sale of securities available for sale $ 0 3,401,000 $ 14,005,000
Gross gains from sale of available for sale securities   95,000  
Gross losses from sale of available for sale securities   $ 45,000  
Number of securities in an unrealized loss position | security 122 87  
Available-for-sale Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost $ 48,320,000    
Securities available for sale, at fair value 48,787,000    
Proceeds from the sale of securities available for sale 0 $ 3,400,000  
Mortgage-backed securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 993,548,000 952,992,000  
Securities available for sale, at fair value 988,367,000 951,861,000  
Equity Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 417,000 397,000  
Securities available for sale, at fair value 658,000 $ 549,000  
Private Label Mortgage-Backed Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost $ 99,000    
Number of securities in an unrealized loss position | security 2    
Unrealized losses $ 2,000