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Note 11 - Stock-based Compensation (Details Textual) - USD ($) $ in Millions |  |  |  |  |  |  |  | 8 Months Ended | 9 Months Ended | 12 Months Ended | 36 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
May 27, 2021 | Aug. 27, 2020 | Jun. 01, 2020 | May 23, 2019 | Oct. 25, 2018 | May 24, 2018 | Jun. 01, 2014 | May 31, 2019 | May 31, 2021 | May 31, 2022 | Dec. 31, 2021 | May 31, 2021 | Dec. 31, 2020 | May 31, 2020 | Dec. 31, 2019 | May 31, 2019 | Dec. 31, 2015 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) |  |  |  |  |  |  |  |  |  |  | 187,000 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 0 |  |  |
Stock Issued During Period, Shares, Issued for Services (in shares) |  |  |  |  |  |  |  |  |  |  | 10,489 |  | 17,716 |  | 11,905 |  |  |
Mercantile Plans [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 0 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value |  |  |  |  |  |  |  |  |  |  | $ 0.1 |  |  |  |  |  |  |
Firstbank Corporation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) |  |  |  |  |  |  | 282,200 |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares (in shares) |  |  |  |  |  |  | 258,400 |  |  |  |  |  |  |  |  |  | 23,800 |
First Bank Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares for Maximum Vesting Rights (in shares) |  |  |  |  |  |  |  |  |  |  | 50 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Minimum Number of Shares for Minimum Vesting Rights (in shares) |  |  |  |  |  |  |  |  |  |  | 50 |  |  |  |  |  |  |
First Bank Employees [Member] | Grant Per Annum Starting One Year From Date of Grant [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |
First Bank Employees [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |
First Bank Employees [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |
Director [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Issued for Services (in shares) | 10,000 |  | 16,000 | 12,000 |  | 11,000 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  |  |  |  |  | $ 0.3 |  | $ 0.4 |  | $ 0.4 |  |
Director [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  |  |  | $ 0.3 |  |  |  |  |  |  |  |
Bank Board Members [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Issued for Services (in shares) |  |  |  |  | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  | $ 0.1 | $ 0.1 |  |  |  |  |  |  |  |  |
Corporate and Bank Board Members [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Issued for Services (in shares) |  | 2,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |
Restricted Stock [Member] | Mercantile Plans [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 163,000 |  | 121,518 |  | 84,596 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 11,873 |  | 35,997 |  | 3,531 |  |  |
Share-based Payment Arrangement, Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 110.00%
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7 years
Performance Shares [Member] | Executive Officer [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 22,703 |  | 31,295 |  | 23,596 |  |  |
Performance Shares [Member] | Executive Officer [Member] | The 2020 Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 11,454 |  |  |  |  |  |  |
Performance Shares [Member] | Executive Officer [Member] | The 2019 Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 23,053 |  | 13,709 |  |  |  |  |
Performance Shares [Member] | Executive Officer [Member] | The 2018 Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 22,836 |  | 14,681 |  |  |  |  |

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- Definition Represents the benchmark for the minimum number of shares awarded required for minimum vesting rights. + References No definition available. + Details
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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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