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Loans and Allowance for Credit Losses - Additional Information (Details) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2021 USD ($) alternative Loan_modified | Mar. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) Loan_modified | Jan. 01, 2020 USD ($) | Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Accrued interest on loans | $ 15,600,000 |  | $ 15,900,000 |  |
Minimum value of loans individually measured for impairment | $ 250,000 |  |  |  |
Number of alternatives for measuring impaired loans receivable | alternative | 3 |  |  |  |
Allowance for credit losses | $ 55,418,000 | $ 32,876,000 | 41,910,000 |  | $ 26,911,000
Financing receivable modifications performing recorded investment | 4,103,000 | 4,400,000 | 4,373,000 |  |
Interest lost on nonaccrual loans | 1,021,000 | 1,029,000 |  |  |
Troubled debt restructurings balance | 8,689,000 |  | 9,502,000 |  |
Loans receivable, modifications, allowance | 929,000 |  | 1,016,000 |  |
Real estate acquired through foreclosure | 13,339,000 |  | 2,489,000 |  |
Mortgage loans secured by real estate In foreclosure | $ 634,000 |  | $ 713,000 |  |
Financial Asset, Equal to or Greater than 90 Days Past Due |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Troubled debt restructurings balance | Loan_modified | 0 |  | 0 |  |
Accounting Standards Update 2016-13 |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | $ 28,583,000 |
Accounting Standards Update 2016-13 | Impact of ASU |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 1,672,000 |
Accounting Standards Update 2016-13 | Financial Asset Acquired with Credit Deterioration |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 1,198,000 |
Accounting Standards Update 2016-13 | Financial Asset Acquired with Credit Deterioration | Impact of ASU |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 833,000 |
Commercial Real Estate |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Debt coverage ratio | 1.20x |  |  |  |
Amortization period of loans | 20 years |  |  |  |
Commercial Real Estate | Accounting Standards Update 2016-13 |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 11,739,000 |
Commercial Real Estate | Accounting Standards Update 2016-13 | Impact of ASU |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 541,000 |
Commercial Real Estate | Accounting Standards Update 2016-13 | Financial Asset Acquired with Credit Deterioration |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 818,000 |
Commercial Real Estate | Accounting Standards Update 2016-13 | Financial Asset Acquired with Credit Deterioration | Impact of ASU |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 459,000 |
Commercial Real Estate | Minimum |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Maximum loan-to-value ratio (in hundredths) | 65.00% |  |  |  |
Commercial Real Estate | Maximum |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Maximum loan-to-value ratio (in hundredths) | 80.00% |  |  |  |
1-4 Family Residential Properties |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Period when loans charged-down | 180 days |  |  |  |
1-4 Family Residential Properties | Accounting Standards Update 2016-13 |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 2,142,000 |
1-4 Family Residential Properties | Accounting Standards Update 2016-13 | Impact of ASU |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 756,000 |
1-4 Family Residential Properties | Accounting Standards Update 2016-13 | Financial Asset Acquired with Credit Deterioration |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | 48,000 |
1-4 Family Residential Properties | Accounting Standards Update 2016-13 | Financial Asset Acquired with Credit Deterioration | Impact of ASU |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses |  |  |  | $ 42,000 |
Motels and Hotels Loans |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | $ 126,000,000.0 |  |  |  |
Non-residential Buildings |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | 426,300,000 |  |  |  |
Residential Buildings and Dwellings |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | 319,800,000 |  |  |  |
Nursing Care Facilities |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | 109,800,000 |  |  |  |
Other Gambling Industries |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | 121,300,000 |  |  |  |
Real Estate Loan |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Financing receivable modifications performing recorded investment | 4,031,000 |  | $ 4,313,000 |  |
Troubled debt restructurings balance | $ 5,975,000 |  | 6,773,000 |  |
Unsecured Open-end Loans |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Period when loans charged-down | 180 days |  |  |  |
Other Secured Loans |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Period when loans charged-down | 120 days |  |  |  |
Agricultural |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | $ 390,700,000 |  | 391,700,000 |  |
Increase (decrease) in accounts and notes receivable | $ (1,000,000.0) |  |  |  |
Maximum loan-to-value ratio (in hundredths) | 65.00% |  |  |  |
Amortization period of loans | 25 years |  |  |  |
Loans receivable, time period | 1 year |  |  |  |
Agricultural | Corn and Other Grain Farming |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | $ 278,200,000 |  | 308,200,000 |  |
Increase (decrease) in accounts and notes receivable | 30,000,000.0 |  |  |  |
Agricultural | Real Estate Loan |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Allowance for credit losses | $ 1,628,000 | 1,335,000 | 1,084,000 |  | 1,093,000
Commercial and Industrial Loans |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Maximum loan-to-value ratio (in hundredths) | 80.00% |  |  |  |
Amortization period of loans | 7 years |  |  |  |
Loans receivable, time period | 1 year |  |  |  |
Allowance for credit losses | $ 18,954,000 | $ 11,294,000 | 13,485,000 |  | $ 9,273,000
Financing receivable modifications performing recorded investment | 55,000 |  | 58,000 |  |
Troubled debt restructurings balance | $ 2,362,000 |  | $ 2,389,000 |  |
Residential Properties | Real Estate Loan |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Maximum loan-to-value ratio (in hundredths) | 80.00% |  |  |  |
Amortization period of loans | 25 years |  |  |  |
Balloon period | five years |  |  |  |

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