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New Accounting Standards (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2020
Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Allowance for credit losses on loans   $ 238,050 $ 68,244      
Unfunded commitments, allowance for credit loss   22,400 8,400      
Undivided profits   901,006 848,848      
Unrealized holding gains (losses) arising during the period on available-for-sale securities   107,382 76,109 $ (13,626)    
Right-of-use lease assets   31,348 40,675      
Lease liability included in other liabilities   $ 31,433 $ 40,854      
Impact of ASU 2016-13 adoption            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Allowance for credit losses on loans         $ 151,400  
Unfunded commitments, allowance for credit loss         24,000  
Debt securities, allowance for credit loss         742  
Undivided profits         $ 128,100  
Accounting Standards Update 2017-12            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Fair value of HTM municipal bonds $ 216,400          
Unrealized holding gains (losses) arising during the period on available-for-sale securities $ 2,500          
Accounting Standards Update 2016-02            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Right-of-use lease assets           $ 32,800
Lease liability included in other liabilities           $ 32,800