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Held to Maturity Debt Securities - Additional Information (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Dec. 31, 2018
USD ($)
Jan. 01, 2020
USD ($)
Dec. 31, 2017
USD ($)
Schedule of Held-to-maturity Securities [Line Items]          
Allowance for credit losses $ (78) $ 0   $ 70  
Retained earnings 718,090 695,273      
Held to maturity securities at carrying value 416,100 428,000      
Investment securities held to maturity 450,965 453,629      
Fair value 472,451 467,966      
Allowance for credit losses 101,466 55,525 $ 55,562   $ 60,195
Proceeds from maturities, calls and paydowns of held to maturity debt securities $ 62,051 $ 42,696 39,534    
Number of securities in an unrealized loss position | security 7 35      
AAA          
Schedule of Held-to-maturity Securities [Line Items]          
Investment securities held to maturity $ 65,492 $ 56,033      
Amount of total portfolio 15.00%        
AA          
Schedule of Held-to-maturity Securities [Line Items]          
Investment securities held to maturity $ 314,410 333,450      
Amount of total portfolio 70.00%        
A          
Schedule of Held-to-maturity Securities [Line Items]          
Investment securities held to maturity $ 59,748 62,652      
Amount of total portfolio 13.00%        
Fitch, A Rating Or Not Rated          
Schedule of Held-to-maturity Securities [Line Items]          
Amount of total portfolio 2.00%        
Held-to-maturity Securities          
Schedule of Held-to-maturity Securities [Line Items]          
Recognized gain on calls of securities held to maturity portfolio $ 81 72 10    
Recognized loss on calls of securities held to maturity portfolio 0 0 1    
Proceeds from maturities, calls and paydowns of held to maturity debt securities 49,300 33,900 $ 32,000    
Mortgage-backed securities          
Schedule of Held-to-maturity Securities [Line Items]          
Allowance for credit losses 0 0      
Investment securities held to maturity 62        
Fair value 64 122      
Allowance for credit losses 78        
Mortgage-backed securities | AAA          
Schedule of Held-to-maturity Securities [Line Items]          
Investment securities held to maturity 62 118      
Mortgage-backed securities | AA          
Schedule of Held-to-maturity Securities [Line Items]          
Investment securities held to maturity 0 0      
Mortgage-backed securities | A          
Schedule of Held-to-maturity Securities [Line Items]          
Investment securities held to maturity $ 0 $ 0      
Accounting Standards Update 2016-13          
Schedule of Held-to-maturity Securities [Line Items]          
Allowance for credit losses       70  
Retained earnings       8,300  
Allowance for credit losses       7,900  
Accounting Standards Update 2016-13, Effect On Debt Securities          
Schedule of Held-to-maturity Securities [Line Items]          
Retained earnings       $ 52