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Investment Securities - Textual (Details) - USD ($)
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]      
Held-to-maturity, interest receivable $ 3,400,000    
Available-for-sale, interest receivable 20,200,000    
Available-for-sale securities 4,528,348,000   $ 3,473,598,000
Securities valued at less than historical cost, amount $ 2,422,837,000    
Securities valued at less than historical cost (as percent) 53.50%    
Decrease in available-for-sale provision for credit losses $ 2,142,000 $ (104,000)  
Securities pledged as collateral 2,810,000,000   $ 2,010,000,000.00
Realized gains 5,500,000 32,100,000  
Realized losses 13,000 (2,080)  
Investments sold, amount $ 135,700,000 $ 1,100,000,000  
Income tax benefit related to security gains (losses) (as percent) 26.135% 26.135%