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Investment Securities - Textual (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]                
Debt securities, available-for-sale, transfer to held-to-maturity $ 500,800,000              
Amortization of securities transferred from available for sale to held to maturity 1,106,000 $ 0 $ 1,106,000 $ 0        
Available-for-sale, transfer to held-to-maturity, gain (loss) 0              
Held to maturity mortgage backed securities 1,518,076,000   1,518,076,000     $ 335,946,000    
Available for sale mortgage backed securities 6,838,743,000   6,838,743,000     3,397,043,000    
Held-to-maturity, interest receivable 8,300,000   8,300,000          
Available-for-sale, interest receivable 20,900,000   20,900,000          
Available-for-sale securities 6,822,203,000   6,822,203,000     3,473,598,000    
Securities valued at less than historical cost, amount $ 3,840,900,000   $ 3,840,900,000          
Securities valued at less than historical cost (as percent) 56.30%   56.30%          
Decrease in available-for-sale provision for credit losses $ 0 602,000 $ (312,000) 836,000        
Provision for credit losses   599,000   835,000        
Allowance for credit losses on HTM 1,279,000 373,000 1,279,000 373,000 $ 1,132,000 2,915,000 $ 307,000 $ 0
Securities pledged as collateral 3,690,000,000   3,690,000,000     2,010,000,000.00    
Realized gains 5,300,000 22,300,000 15,900,000 54,800,000        
Realized losses (24,500) (1,700) (63,600) $ 4,400        
Investments sold, amount   1,700,000,000 $ 342,600,000          
Income tax benefit related to security gains (losses) (as percent)     26.135% 26.135%        
Commercial Mortgage Backed Securities                
Debt Securities, Available-for-sale [Line Items]                
Held to maturity mortgage backed securities 5,400,000   $ 5,400,000     7,700,000    
Available for sale mortgage backed securities 1,560,000,000   1,560,000,000     406,100,000    
Residential Mortgage Backed Securities                
Debt Securities, Available-for-sale [Line Items]                
Held to maturity mortgage backed securities 52,500,000   52,500,000     14,700,000    
Available for sale mortgage backed securities 2,860,000,000   2,860,000,000     988,800,000    
State and political subdivisions                
Debt Securities, Available-for-sale [Line Items]                
Held to maturity mortgage backed securities 1,210,287,000   1,210,287,000     312,416,000    
Available for sale mortgage backed securities 1,574,674,000   1,574,674,000     1,416,136,000    
Available-for-sale securities 1,575,208,000   1,575,208,000     1,470,723,000    
Securities valued at less than historical cost, amount 992,492,000   992,492,000          
Allowance for credit losses on HTM 1,196,000 $ 73,000 1,196,000 $ 73,000 $ 871,000 $ 2,307,000 $ 95,000 $ 0
State and political subdivisions | Other Credit Enhancement or Insurance                
Debt Securities, Available-for-sale [Line Items]                
Held to maturity mortgage backed securities 531,863,000   531,863,000          
Allowance for credit losses on HTM $ 0   $ 0