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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