Loans, Allowance for Loan Losses and Impaired Loans - Schedule of Purchased Loans, Impaired and Non-Impaired (Details) - Acquired Loans - USD ($) $ in Thousands	Jul. 01, 2019	Oct. 19, 2018
F. & M. Bancorp. of Tomah, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required cash flows at acquisition	$ 130,274
Non-accretable difference (expected losses and foregone interest)	(2,728)
Cash flows expected to be collected at acquisition	127,546
Accretable yield	(814)
Fair value of acquired loans at acquisition	126,732
F. & M. Bancorp. of Tomah, Inc. | Performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required cash flows at acquisition	111,919
Non-accretable difference (expected losses and foregone interest)	0
Cash flows expected to be collected at acquisition	111,919
Accretable yield	(814)
Fair value of acquired loans at acquisition	111,105
F. & M. Bancorp. of Tomah, Inc. | Receivables Acquired with Deteriorated Credit Quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required cash flows at acquisition	18,355
Non-accretable difference (expected losses and foregone interest)	(2,728)
Cash flows expected to be collected at acquisition	15,627
Accretable yield	0
Fair value of acquired loans at acquisition	$ 15,627
United Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required cash flows at acquisition		$ 203,583
Non-accretable difference (expected losses and foregone interest)		(2,704)
Cash flows expected to be collected at acquisition		200,879
Accretable yield		(1,020)
Fair value of acquired loans at acquisition		199,859
United Bank | Performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required cash flows at acquisition		183,317
Non-accretable difference (expected losses and foregone interest)		0
Cash flows expected to be collected at acquisition		183,317
Accretable yield		(1,020)
Fair value of acquired loans at acquisition		182,297
United Bank | Receivables Acquired with Deteriorated Credit Quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required cash flows at acquisition		20,266
Non-accretable difference (expected losses and foregone interest)		(2,704)
Cash flows expected to be collected at acquisition		17,562
Accretable yield		0
Fair value of acquired loans at acquisition		$ 17,562