GOODWILL, CORE DEPOSIT INTANGIBLES AND OTHER INTANGIBLE ASSETS (Book Value, Fair Value, and Valuation Allowance of Mortgage and Commercial Servicing Rights) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Mortgage Servicing Rights 15-year Tranche
Finite-Lived Intangible Assets [Line Items]
Book Value	$ 1,522,000	$ 1,482,000
Fair Value	1,100,000	1,368,000
Impairment	422,000	114,000
Mortgage Servicing Rights 15-year Tranche | First Bank & Trust
Finite-Lived Intangible Assets [Line Items]
Book Value	1,522,000	1,482,000
Fair Value	1,100,000	1,368,000
Impairment	422,000	114,000
Mortgage Servicing Rights 30-year Tranche
Finite-Lived Intangible Assets [Line Items]
Book Value	5,445,000	5,050,000
Fair Value	4,089,000	4,253,000
Impairment	1,356,000	797,000
Mortgage Servicing Rights 30-year Tranche | First Bank & Trust
Finite-Lived Intangible Assets [Line Items]
Book Value	5,445,000	5,050,000
Fair Value	4,089,000	4,253,000
Impairment	1,356,000	797,000
Commercial Servicing Rights less than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	87,000	129,000
Fair Value	203,000	285,000
Impairment	0	0
Commercial Servicing Rights less than 20 Years | Premier Valley Bank
Finite-Lived Intangible Assets [Line Items]
Book Value	0	1,000
Fair Value	7,000	13,000
Impairment	0	0
Commercial Servicing Rights less than 20 Years | Wisconsin Bank & Trust
Finite-Lived Intangible Assets [Line Items]
Book Value	87,000	128,000
Fair Value	196,000	272,000
Impairment	0	0
Commercial Servicing Rights more than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	776,000	986,000
Fair Value	1,085,000	1,309,000
Impairment	0	0
Commercial Servicing Rights more than 20 Years | Premier Valley Bank
Finite-Lived Intangible Assets [Line Items]
Book Value	86,000	135,000
Fair Value	143,000	161,000
Impairment	0
Commercial Servicing Rights more than 20 Years | Wisconsin Bank & Trust
Finite-Lived Intangible Assets [Line Items]
Book Value	690,000	851,000
Fair Value	942,000	1,148,000
Impairment	$ 0	$ 0