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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 |  |  |  |  |  |

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