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Basis of Accounting and Consolidation - Additional Information (Details)
 12 Months Ended
Sep. 25, 2020
USD ($)
$ / shares
shares
Jan. 02, 2020
USD ($)
Dec. 31, 2020
USD ($)
TrustAccount
$ / shares
shares
Dec. 31, 2019
USD ($)
TrustAccount
$ / shares
shares
Dec. 31, 2018
USD ($)
shares
Apr. 21, 2020
USD ($)
Jan. 01, 2020
USD ($)
Dec. 31, 2017
USD ($)
Summary Of Significant Accounting Policies [Line Items]                
Common stock, par value (in dollars per share) | $ / shares     $ 4 $ 4        
Loans acquired           $ 183,000,000    
Federal funds selling period     1 day          
Captive maximum gross premiums     $ 2,300,000          
Number of trust accounts managed or administered | TrustAccount     1,411 1,635        
Value of trust accounts     $ 1,563,000,000.0 $ 1,487,500,000        
Stock plans, term     10 years          
Maximum number of shares to be issued in stock incentive plan (in shares) | shares     149,983          
RSA/RSU, grants in period, gross | shares     25,950 26,700 28,700      
Employee discount for employee stock purchase plan     5.00%          
Employee stock purchase plan, number of shares authorized | shares     600,000          
Stock issued during period, shares, employee stock purchase plans | shares     11,037 8,899        
Right of use lease assets     $ 17,209,000 $ 17,006,000        
Lease liabilities     17,351,000 17,007,000        
Cumulative change in accounting principal     568,228,000 526,609,000 $ 475,864,000     $ 307,964,000
Allowance for credit losses on loans     $ 41,910,000 26,911,000 $ 26,189,000     $ 19,977,000
ASU 2016-02                
Summary Of Significant Accounting Policies [Line Items]                
Change in accounting principle, accounting standards update, adopted [true false]     true          
Change in accounting principle, accounting standards update, adoption date     Jan. 01, 2019          
Right of use lease assets     $ 14,100,000          
Lease liabilities     $ 14,100,000          
ASU 2016-13                
Summary Of Significant Accounting Policies [Line Items]                
Allowance for credit losses on loans             $ 28,583,000  
Off-balance sheet, credit loss, liability             69,000  
Allowance for credit losses, held-to-maturity debt securities             0  
Amortized cost of PCD assets reflected the addition to allowance for credit losses   $ 833,000            
ASU 2016-13 | Impact of ASU                
Summary Of Significant Accounting Policies [Line Items]                
Allowance for credit losses on loans             1,672,000  
Off-balance sheet, credit loss, liability             69,000  
ASU 2016-13 | Cumulative Change in Accounting Principal, Adjustment                
Summary Of Significant Accounting Policies [Line Items]                
Cumulative change in accounting principal       (717,000)     $ (717,000)  
Allowance for credit losses on loans       $ 1,672,000        
Stock Unit Awards                
Summary Of Significant Accounting Policies [Line Items]                
RSA/RSU, grants in period, gross | shares     25,950 26,700 28,700      
Minimum                
Summary Of Significant Accounting Policies [Line Items]                
Certificates of deposit investments original maturities period     3 years          
Maximum                
Summary Of Significant Accounting Policies [Line Items]                
Certificates of deposit investments original maturities period     5 years          
Intangible assets amortization period     15 years          
LINCO Bancshares, Inc.                
Summary Of Significant Accounting Policies [Line Items]                
Percentage of issued and outstanding shares acquired 100.00%              
Common stock, par value (in dollars per share) | $ / shares $ 1.00              
Share Price | $ / shares $ 4.00              
Consideration payable in cash $ 103,500,000              
Consideration payable in shares | shares 1,262,246              
Dividends $ 13,000,000