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Available for Sale Debt Securities - Additional Information (Detail)
12 Months Ended
Dec. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Dec. 31, 2018
USD ($)
Debt Securities, Available-for-sale [Line Items]      
Securities available for sale at carrying value $ 618,000,000.0 $ 536,400,000  
Amortized cost 1,073,570,000 965,135,000  
Fair value 1,105,489,000 976,919,000  
Proceeds from sales of available for sale debt securities $ 13,905,000 $ 0 $ 2,212,000
Number of securities in an unrealized loss position | security 42 50  
Mortgage-backed securities      
Debt Securities, Available-for-sale [Line Items]      
Amortized cost $ 910,393,000 $ 936,196,000  
Fair value 938,413,000 $ 947,430,000  
Private Label Mortgage-Backed Securities      
Debt Securities, Available-for-sale [Line Items]      
Amortized cost $ 17,445    
Number of securities in an unrealized loss position | security 1    
Unrealized losses $ (1,300)    
Agency Obligations, Collateralized Mortgage Backed Securities And Asset-Backed Securities      
Debt Securities, Available-for-sale [Line Items]      
Amortized cost 963,700,000    
Fair value 993,300,000    
Available-for-sale Securities      
Debt Securities, Available-for-sale [Line Items]      
Amortized cost 109,881,000    
Fair value 112,237,000    
Proceeds from sales of available for sale debt securities $ 13,900,000