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Basis of Accounting and Consolidation - Additional Information (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 10, 2021
Jul. 28, 2021
Oct. 06, 2020
Jan. 01, 2020
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Jun. 30, 2021
Jun. 30, 2020
Apr. 21, 2020
Dec. 31, 2019
Basis Of Accounting And Consolidation [Line Items]                          
Common stock, par value (in dollars per share)         $ 4   $ 4   $ 4        
Assets         $ 6,043,359,000   $ 6,043,359,000   $ 4,726,348,000        
Loans         3,891,889,000   3,891,889,000   3,094,585,000        
Deposits $ 215,000,000       $ 4,988,562,000   $ 4,988,562,000   3,692,784,000     $ 60,000,000  
Loans acquired 208,000,000                     $ 183,000,000  
Core deposit intangible asset $ 4,848,000                        
Amortization period 10 years                        
Stock plans, term             10 years            
Maximum number of shares to be issued in stock incentive plan (in shares)         399,983   399,983            
Employee discount for employee stock purchase plan             5.00%            
Employee stock purchase plan, number of shares authorized         600,000   600,000            
Stock issued during period, shares, employee stock purchase plans         2,700 3,494 8,439 11,037          
Captive maximum gross premiums             $ 2,300,000            
Cumulative change in accounting principal         $ 627,225,000 $ 561,009,000 627,225,000 $ 561,009,000 568,228,000 $ 616,066,000 $ 549,273,000   $ 526,609,000
Allowance for credit losses on loans         53,983,000 $ 41,915,000 $ 53,983,000 $ 41,915,000 $ 41,910,000 $ 54,597,000 $ 38,381,000   26,911,000
ASU 2016-13                          
Basis Of Accounting And Consolidation [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                          
Basis Of Accounting And Consolidation [Line Items]                          
Allowance for credit losses on loans       1,672,000                  
Off-balance sheet, credit loss, liability       69,000                  
Cumulative Effect, Period of Adoption, Adjustment | ASU 2016-13                          
Basis Of Accounting And Consolidation [Line Items]                          
Cumulative change in accounting principal       $ 717,000                 (717,000)
Allowance for credit losses on loans                         $ 1,672,000
Stock Options                          
Basis Of Accounting And Consolidation [Line Items]                          
Common stock awarded in SI plan (in shares)             0            
Restricted Stock                          
Basis Of Accounting And Consolidation [Line Items]                          
Common stock awarded in SI plan (in shares)             27,750   25,200        
Restricted Stock Units (RSUs)                          
Basis Of Accounting And Consolidation [Line Items]                          
Common stock awarded in SI plan (in shares)             35,400   16,950        
3.95% Subordinated Notes due 2030                          
Basis Of Accounting And Consolidation [Line Items]                          
Aggregate principal amount     $ 96,000,000.0                    
Fixed-to-Floating Rate     3.95%                    
Maturity date     Oct. 15, 2030                    
Redemption description     The Company may, beginning with the interest payment date of October 15, 2025, and on any interest payment date thereafter, redeem the Notes, in whole or in part, at a redemption price equal to 100% of the principal amount of the Notes to be redeemed plus accrued and unpaid interest to but excluding the date of redemption. The Company may also redeem the Notes at any time, including prior to October 15, 2025, at the Company’s option, in whole but not in part, if: (i) a change or prospective change in law occurs that could prevent the Company from deducting interest payable on the Notes for U.S. federal income tax purposes; (ii) a subsequent event occurs that could preclude the Notes from being recognized as Tier 2 capital for regulatory capital purposes; or (iii) the Company is required to register as an investment company under the Investment Company Act of 1940, as amended; in each case, at a redemption price equal to 100% of the principal amount of the Notes plus any accrued and unpaid interest to but excluding the redemption date.                    
3.95% Subordinated Notes due 2030 | SOFR                          
Basis Of Accounting And Consolidation [Line Items]                          
Description floating rate basis     three-month Term SOFR                    
Basis points     3.83%                    
Delta Bancshares Company                          
Basis Of Accounting And Consolidation [Line Items]                          
Percentage of issued and outstanding shares acquired   100.00%                      
Common stock, par value (in dollars per share)   $ 10.00                      
Share Price   $ 4.00                      
Consideration payable in cash   $ 15,200,000                      
Consideration payable in shares   2,282,511                      
Assets         668,000,000   $ 668,000,000            
Loans         448,000,000   448,000,000            
Deposits         $ 511,000,000   $ 511,000,000