Annual report pursuant to Section 13 and 15(d)

Securities - Additional Information (Detail)

v3.20.4
Securities - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items]        
Reduction in securities available for sale $ (753,781,000) $ (634,477,000)    
Reduction to retained earnings (114,621,000) (100,551,000)    
Net gains (losses) in earnings resulting from sale of securities 15,712,000 1,295,000 $ (870,000)  
Net unrealized gains (losses) recorded in comprehensive income 15,300,000 586,000 (413,000)  
Gross realized losses included in earnings   64,000 957,000  
Gain (Loss) on Sale of Investments     (428,000)  
Available for sale securities pledged as collateral $ 27,300,000 $ 30,200,000    
Minimum | U.S. Government and its Agencies        
Debt Securities, Available-for-sale [Line Items]        
Holdings of securities as percentage of share holders' equity 10.00%      
ASU 2016-01        
Debt Securities, Available-for-sale [Line Items]        
Reduction in securities available for sale       $ 529,000
Reduction to retained earnings       382,000
Increase in net deferred tax asset       $ 147,000
Net unrealized gains (losses) recorded in comprehensive income     $ (529,000)