SHORT AND LONG-TERM BORROWINGS - Junior Subordinated Debentures (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Par	$ 38,249,000
Unamortized discount	(8,153,000)
Carrying Value	30,096,000	$ 29,616,000
2005 Mid-Wisconsin Financial Services, Inc.
Debt Instrument [Line Items]
Par	10,310,000
Unamortized discount	(3,371,000)
Carrying Value	$ 6,939,000	$ 6,739,000
Interest rate	4.22%	3.02%
2006 Baylake Corp.
Debt Instrument [Line Items]
Par	$ 16,598,000
Unamortized discount	(4,120,000)
Carrying Value	$ 12,478,000	$ 12,242,000
Interest rate	4.15%	3.04%
2004 First Menasha Bancshares, Inc.
Debt Instrument [Line Items]
Par	$ 5,155,000
Unamortized discount	(662,000)
Carrying Value	$ 4,493,000	$ 4,449,000
Interest rate	5.58%	4.39%
2004 Nicolet Bankshares Statutory Trust
Debt Instrument [Line Items]
Par	$ 6,186,000
Unamortized discount	0
Carrying Value	$ 6,186,000	$ 6,186,000
Interest rate	8.00%
London Interbank Offered Rate (LIBOR) \| 2005 Mid-Wisconsin Financial Services, Inc.
Debt Instrument [Line Items]
Basis spread rate	1.43%
London Interbank Offered Rate (LIBOR) \| 2006 Baylake Corp.
Debt Instrument [Line Items]
Basis spread rate	1.35%
London Interbank Offered Rate (LIBOR) \| 2004 First Menasha Bancshares, Inc.
Debt Instrument [Line Items]
Basis spread rate	2.79%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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-Name Accounting Standards Codification
-Topic 210
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount, after accumulated amortization, of debt discount.

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments.

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