NATURE OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($)	Mar. 31, 2022 USD ($)	Mar. 31, 2022 USD ($) branch	Mar. 31, 2022 USD ($) operating_segment	Mar. 31, 2022 USD ($) reporting_unit	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Liquidating investment period (in years)	10 years
Other investments	$ 15,084	$ 15,084	$ 15,084	$ 15,084	$ 15,084	$ 15,084		$ 15,305
Loans charged off past due more than days	180 days
Closed end loan charged off past due more than days	120 days
Number of operating segments				1		1
Number of reporting units | reporting_unit					1
Carrying amount of investment		163					$ 0
Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount of investment		3,833
FHLB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	$ 7,650	7,650	7,650	$ 7,650	$ 7,650	$ 7,650		7,877
Federal Reserve Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	5,205	5,205	5,205	5,205	5,205	5,205		5,200
Bankers Bank Stock
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	$ 2,229	$ 2,229	$ 2,229	$ 2,229	$ 2,229	$ 2,229		$ 2,228
Commercial Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income discontinued over delinquent days	90 days
Closed End Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income discontinued over delinquent days	120 days
Real Estate and Open Ended Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income discontinued over delinquent days	180 days
Wisconsin and Minnesota
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of branch locations | branch			25