NATURE OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2021 USD ($) branch	Mar. 31, 2021 USD ($) operating_segment	Mar. 31, 2021 USD ($) reporting_unit	Mar. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	$ 15,069	$ 15,069	$ 15,069	$ 15,069	$ 15,069	$ 14,948
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
FHLB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	$ 8,041	8,041	$ 8,041	$ 8,041	$ 8,041	8,103
Federal Reserve Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	5,181	5,181	5,181	5,181	5,181	5,170
Bankers Bank Stock
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other investments	$ 1,847	$ 1,847	$ 1,847	$ 1,847	$ 1,847	$ 1,675
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