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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
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 27, 2020 | Jun. 01, 2020 | May 23, 2019 | Oct. 25, 2018 | May 24, 2018 | May 25, 2017 | Jun. 01, 2014 | May 31, 2019 | May 31, 2021 | May 31, 2021 | Dec. 31, 2020 | May 31, 2020 | Dec. 31, 2019 | May 31, 2019 | Dec. 31, 2018 | May 31, 2018 | Dec. 31, 2015 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) |  |  |  |  |  |  |  |  |  |  | 361,000 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 0 |  |  |
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) |  | 16,000 | 12,000 |  | 11,000 | 12,000 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  |  |  |  |  | $ 0.4 |  | $ 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) | 2,000 |  |  | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  | $ 0.1 |  |  |  |  |  |  |  |  |  |
Bank Board Members [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  |  | $ 0.1 |  |  |  |  |  |  |  |  |
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) |  |  |  |  |  |  |  |  |  |  | 121,518 |  | 84,596 |  | 91,400 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares) |  |  |  |  |  |  |  |  |  |  | 35,997 |  | 3,531 |  | 12,278 |  |  |
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) |  |  |  |  |  |  |  |  |  |  | 31,295 |  | 23,596 |  | 24,633 |  |  |
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) |  |  |  |  |  |  |  |  |  |  | 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) |  |  |  |  |  |  |  |  |  |  | 14,681 |  |  |  |  |  |  |

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