GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Book value, fair value of commercial serving rights and impairment) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Impairment	$ 589,000	$ 58,000
15 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	3,626,000	3,880,000
Fair Value	6,167,000	6,301,000
Impairment	125,000	20,000
30 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	24,493,000	25,541,000
Fair Value	37,360,000	42,379,000
Impairment	522,000	38,000
Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	249,000	295,000
Fair Value	452,000	491,000
Impairment	0	0
More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,247,000	1,414,000
Fair Value	1,474,000	1,643,000
Impairment	0	0
Dubuque Bank and Trust Company | 15 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,982,000	2,195,000
Fair Value	4,648,000	4,636,000
Impairment	0	0
Dubuque Bank and Trust Company | 30 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	19,031,000	20,025,000
Fair Value	32,420,000	36,901,000
Impairment	0	0
First Bank & Trust | 15 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,644,000	1,685,000
Fair Value	1,519,000	1,665,000
Impairment	125,000	20,000
First Bank & Trust | 30 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	5,462,000	5,516,000
Fair Value	4,940,000	5,478,000
Impairment	522,000	38,000
Citywide Banks | Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	0	1,000
Fair Value	0	6,000
Impairment	0	0
Citywide Banks | More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	0	18,000
Fair Value	0	20,000
Impairment	0	0
Premier Valley Bank | Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	37,000	45,000
Fair Value	65,000	74,000
Impairment	0	0
Premier Valley Bank | More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	168,000	178,000
Fair Value	178,000	184,000
Impairment	0	0
Wisconsin Bank & Trust | Less Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	212,000	249,000
Fair Value	387,000	411,000
Impairment	0	0
Wisconsin Bank & Trust | More Than 20 Years
Finite-Lived Intangible Assets [Line Items]
Book Value	1,079,000	1,218,000
Fair Value	1,296,000	1,439,000
Impairment	$ 0	$ 0