GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Book value, fair value of commercial serving rights and impairment) (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2020	Jun. 30, 2019	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Impairment	$ 2,000,000.0
15 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,478,000		$ 1,482,000
Fair Value	954,000		1,368,000
Impairment	524,000		114,000
30 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	5,181,000		5,050,000
Fair Value	3,171,000		4,253,000
Impairment	2,010,000		797,000
Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	133,000		129,000
Fair Value	249,000		285,000
Impairment	0	$ 0	0
More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	962,000		986,000
Fair Value	1,158,000		1,309,000
Impairment	0	$ 0	0
First Bank & Trust | 15 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,478,000		1,482,000
Fair Value	954,000		1,368,000
Impairment	524,000		114,000
First Bank & Trust | 30 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	5,181,000		5,050,000
Fair Value	3,171,000		4,253,000
Impairment	2,010,000		797,000
Premier Valley Bank | Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,000		1,000
Fair Value	12,000		13,000
Impairment	0		0
Premier Valley Bank | More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	125,000		135,000
Fair Value	145,000		161,000
Impairment	0		0
Wisconsin Bank & Trust | Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	132,000		128,000
Fair Value	237,000		272,000
Impairment	0		0
Wisconsin Bank & Trust | More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	837,000		851,000
Fair Value	1,013,000		1,148,000
Impairment	$ 0		$ 0