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Commitments and Contingencies - Additional Information (Detail) - USD ($)
Nov. 28, 2016
Dec. 31, 2017
Dec. 31, 2016
Contingencies And Commitments [Line Items]      
Commitments to extend credit   $ 924,500,000 $ 844,300,000
Obligations under letters of credit   37,600,000 36,500,000
Reserve for credit risk for undisbursed commitments   6,300,000 $ 6,700,000
Available lines of credit   3,350,000,000  
Litigation settlement payment $ 1,500,000    
Litigation accrual   1,500,000  
Secured [Member]      
Contingencies And Commitments [Line Items]      
Available lines of credit   $ 3,020,000,000