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Commitments and Contingencies - Additional Information (Detail) - USD ($)
Nov. 28, 2016
Oct. 26, 2016
Dec. 31, 2016
Dec. 31, 2015
Contingencies And Commitments [Line Items]        
Commitments to extend credit     $ 844,300,000 $ 744,000,000
Obligations under letters of credit     36,500,000 35,100,000
Reserve for credit risk for undisbursed commitments     6,700,000 $ 7,200,000
Available lines of credit     3,170,000,000  
Litigation settlement payment $ 1,500,000 $ 6,200,000    
Litigation accrual     1,500,000  
Secured [Member]        
Contingencies And Commitments [Line Items]        
Available lines of credit     $ 2,780,000,000