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Commitments and Contingencies - Additional Information (Detail) - USD ($)
Nov. 07, 2018
Sep. 09, 2015
Dec. 31, 2018
Dec. 31, 2017
Contingencies And Commitments [Line Items]        
Commitments to extend credit     $ 1,690,000,000 $ 924,500,000
Obligations under letters of credit     54,300,000 37,600,000
Reserve for credit risk for undisbursed commitments     9,000,000 $ 6,300,000
Available lines of credit     3,960,000,000  
Loss contigency damages sought value   $ 2,000,000    
amount awarded to the plaintiff $ 1,340,000      
insurance policy that covers the amount awarded to the plaintiff 5,000,000      
loss contingency, amount deductible $ 100,000      
Secured [Member]        
Contingencies And Commitments [Line Items]        
Available lines of credit     $ 3,590,000,000