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Investment Securities - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Dec. 31, 2019
USD ($)
security
Dec. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Dec. 31, 2018
USD ($)
Summary of Investment Holdings [Line Items]        
Reclassification of securities $ 382,000,000.0   $ 382,000,000.0  
Increase in capital due to reclassification of held-to-maturity portfolio to available-for-sale portfolio 19,400,000      
Unrealized holding gains, net of tax 13,000,000.0      
Unrealized holding gains (losses) on available-for-sale debt securities   $ 89,204,000 93,479,000 $ 5,792,000
Securities pledged to secure public and other funds, carrying value 2,104,000,000 2,427,000,000 2,104,000,000  
Accrued interest receivable on available-for-sale debt securities   $ 27,000,000.0    
OTTI losses on securities     0 0
Number of securities in security portfolio | security   1,918    
Number of securities in unrealized loss position | security   69    
Total investment securities - available-for-sale 5,385,091,000 $ 5,970,115,000 5,385,091,000  
Unrealized losses 21,660,000 11,385,000 21,660,000  
Equity securities, at fair value 6,842,000 2,547,000 6,842,000  
Gains on equity securities   1,400,000 700,000 100,000
Impairments on equity securities without readily determinable fair value   117,000 0 0
Downward adjustments on equity securities without readily determinable fair value   117,000 0 $ 0
Other assets        
Summary of Investment Holdings [Line Items]        
Equity securities without readily determinable fair value 91,400,000 105,800,000 91,400,000  
Pooled trust preferred securities        
Summary of Investment Holdings [Line Items]        
OTTI losses on securities   $ 0 0  
Number of pooled trust preferred securities | security   2    
Total investment securities - available-for-sale 8,222,000 $ 7,913,000 8,222,000  
Unrealized losses $ 5,589,000 5,850,000 $ 5,589,000  
Indiana        
Summary of Investment Holdings [Line Items]        
Investment securities issued by states and political subdivisions, market value   $ 515,600,000    
State and political subdivision investment, equity percentage   17.30%    
Percentage of municipal bonds rated A or better   99.00%    
Percentage of non rated local interest bonds   1.00%    
Texas        
Summary of Investment Holdings [Line Items]        
Investment securities issued by states and political subdivisions, market value   $ 194,300,000    
State and political subdivision investment, equity percentage   6.50%    
Municipal Bond        
Summary of Investment Holdings [Line Items]        
Number of securities sold | security 6   6  
Investment securities held-to-maturity sold $ 9,700,000      
Gain on investment securities held-to-maturity sold $ 300,000