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Loans - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Apr. 30, 2020
Loans and Leases Receivable Disclosure [Line Items]        
Loans 90 days or more past due and accruing interest $ 0   $ 0  
Internal risk ratings of loans 304,669,000   312,370,000  
Additional committed impared loans 0   0  
Loans modified as troubled debt restructurings 0   0  
Troubled debt restructurings within past twelve months for which there was a default 0 $ 0    
Subsequent event [Member]        
Loans and Leases Receivable Disclosure [Line Items]        
Loans where customers asked us to forebear       $ 18,300,000
Loans where customers asked us to forebear principal amount       $ 461,000
Doubtful [Member]        
Loans and Leases Receivable Disclosure [Line Items]        
Internal risk ratings of loans 0   0  
Other Financial Institutions [Member]        
Loans and Leases Receivable Disclosure [Line Items]        
Participation loans transferred $ 27,750,000   $ 26,153,000