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Note 12 - Stock-based Compensation (Details Textual) - USD ($) $ in Millions |  |  |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
May 23, 2019 | Oct. 25, 2018 | May 24, 2018 | May 25, 2017 | Jun. 06, 2016 | Jun. 01, 2014 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  |  |  |  |  |  | 102,000 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross |  |  |  |  |  |  | 0 | 0 | 0 |  |  |
Mercantile Plans [Member] |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross |  |  |  |  |  |  | 0 | 0 | 0 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value |  |  |  |  |  |  | $ 0.1 |  |  |  |  |
First Bank Plans [Member] |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross |  |  |  |  |  |  | 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 |  |  |  |  |  | 282,200 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares |  |  |  |  |  | 258,400 |  |  |  |  | 23,800 |
First Bank Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period |  |  |  |  |  |  | 10 years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares for Maximum Vesting Rights |  |  |  |  |  |  | 50 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Minimum Number of Shares for Minimum Vesting Rights |  |  |  |  |  |  | 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 |  |  |  |  |  |  | 1 year |  |  |  |  |
First Bank Employees [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period |  |  |  |  |  |  | 5 years |  |  |  |  |
Director [Member] |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Issued for Services | 12,000 |  | 11,000 | 12,000 | 13,000 |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  | $ 0.4 | $ 0.4 | $ 0.3 |  | $ 0.4 |  |  |  |  |
Bank Board Members [Member] |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Issued for Services |  | 1,000 |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  | $ 0.1 |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period |  |  |  |  |  |  | 3 years | 3 years | 3 years | 3 years | 3 years | 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 |  |  |  |  |  |  | 84,596 | 91,400 | 94,592 |  |  |
Share-based Payment Arrangement, Option [Member] |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period |  |  |  |  |  |  |  |  |  | 2 years | 2 years | 2 years
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent |  |  |  |  |  |  |  |  |  | 110.00% | 110.00% | 110.00%
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period |  |  |  |  |  |  |  |  |  | 7 years | 7 years | 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 |  |  |  |  |  |  | 23,596 | 24,633 |  |  |  |

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