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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 |  |  |  |  |  |  |

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