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Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 30, 2020 USD ($) | Sep. 30, 2020 | Dec. 31, 2020 USD ($) | Mar. 31, 2021 USD ($) ft² | Feb. 26, 2021 USD ($) | Dec. 31, 2019 USD ($) | Jan. 01, 2019 USD ($) | Jun. 01, 2014 USD ($)
Number of Unconsolidated Business Trusts Formed to Issue Trust Preferred Securities |  |  | 5 |  |  |  |  |
Financing Receivable, Advances under Paycheck Protection Program Liquidity Facility | $ 43,700 |  | $ 0 |  |  |  |  |
Loans and Leases Receivable, Deferred Income, Total |  |  | $ 4,400 |  |  | $ 500 |  |
Interest Income on Commercial and Mortgage Loans Discontinued Period (Day) |  |  | 90 days |  |  |  |  |
Maximum Delinquency Period for Consumer and Credit Card Loans to be Charged Off (Day) |  |  | 120 days |  |  |  |  |
Repossessed Assets, Total |  |  | $ 700 |  |  | $ 500 |  |
Accounting Standards Update 2016-02 [Member] |  |  |  |  |  |  |  |
Operating Lease, Right-of-Use Asset |  |  |  |  |  |  | $ 1,300 |
Operating Lease, Liability, Total |  |  |  |  |  |  | $ 1,300 |
Banking Office in Hastings, Michigan [Member] | Forecast [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] |  |  |  |  |  |  |  |
Area of Real Estate Property (Square Foot) | ft² |  |  |  | 4,300 |  |  |  |
Disposal Group, Including Discontinued Operation, Deposits |  |  |  | $ 16,000 |  |  |  |
Core Deposits [Member] |  |  |  |  |  |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life (Year) |  | 10 years |  |  |  |  |  |
Commercial Portfolio Segment [Member] |  |  |  |  |  |  |  |
Financing Receivable, Payment Deferrals, Interest-only Amendments, Period (Day) | 90 days |  |  |  |  |  |  |
Financing Receivable, Payment Deferrals, Principal and Interest, Period (Day) | 90 days |  |  |  |  |  |  |
Financing Receivable, Payment Deferrals, Interest Rate | 0.00% |  |  |  |  |  |  |
Financing Receivable, Number of Principal and Interest Payment Deferrals |  |  | 19 |  |  |  |  |
Financing Receivable With Principal and Interest Deferred Payments |  |  | $ 8,000 |  |  |  |  |
Retail Portfolio Segment [Member] |  |  |  |  |  |  |  |
Financing Receivable, Payment Deferrals, Principal and Interest, Period (Day) | 90 days |  |  |  |  |  |  |
Financing Receivable, Number of Principal and Interest Payment Deferrals |  |  | 14 |  |  |  |  |
Financing Receivable With Principal and Interest Deferred Payments |  |  | $ 1,800 |  |  |  |  |
SBA, CARES Act, Paycheck Protection Program [Member] |  |  |  |  |  |  |  |
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums, Total |  |  | $ 15,000 |  |  |  |  |
Number of Loans Originated Under the Paycheck Protection Program |  |  | 2,200 |  |  |  |  |
Financing Receivable, before Allowance for Credit Loss, Total |  |  | $ 554,000 |  |  |  |  |
Financing Receivable, Forgiveness Transactions |  |  | $ 189,000 |  |  |  |  |
Financing Receivable, Forgiveness Transactions, Number of Loans |  |  | 900 |  |  |  |  |
SBA, CARES Act, Paycheck Protection Program [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |
Financing Receivable, Forgiveness Transactions |  |  |  |  | $ 266,000 |  |  |
Financing Receivable, Forgiveness Transactions, Number of Loans |  |  |  |  | 1,400 |  |  |
SBA, CARES Act, Paycheck Protection Program, Second Round [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |
Number of Loans Originated Under the Paycheck Protection Program |  |  |  |  | 900 |  |  |
Financing Receivable, before Allowance for Credit Loss, Total |  |  |  |  | $ 181,000 |  |  |
Firstbank Corporation [Member] |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total |  |  |  |  |  |  |  | $ 1,500,000
Business Combination Branches Acquired |  |  |  |  |  |  |  | 46

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