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Note 18 - Variable Interest Entities - Business Trusts (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Jan. 26, 2016
Preferred securities outstanding | $ 47.6 | $ 46.9 |
Mercantile Bank Capital Trust I [Member] |  |  |
Preferred securities outstanding | $ 21.0 |  | $ 32.0
Mercantile Bank Capital Trust I [Member] | Secured Overnight Financing Rate (SOFR) [Member] |  |  |
Interest rate margin | 2.18% |  |
Firstbank Capital Trust I [Member] |  |  |
Preferred securities outstanding | $ 10.0 |  |
Firstbank Capital Trust I [Member] | Secured Overnight Financing Rate (SOFR) [Member] |  |  |
Interest rate margin | 1.99% |  |
Firstbank Capital Trust II [Member] |  |  |
Preferred securities outstanding | $ 10.0 |  |
Firstbank Capital Trust II [Member] | Secured Overnight Financing Rate (SOFR) [Member] |  |  |
Interest rate margin | 1.27% |  |
Firstbank Capital Trust III [Member] |  |  |
Preferred securities outstanding | $ 7.5 |  |
Firstbank Capital Trust III [Member] | Secured Overnight Financing Rate (SOFR) [Member] |  |  |
Interest rate margin | 1.35% |  |
Firstbank Capital Trust IV [Member] |  |  |
Preferred securities outstanding | $ 7.5 |  |
Firstbank Capital Trust IV [Member] | Secured Overnight Financing Rate (SOFR) [Member] |  |  |
Interest rate margin | 1.35% |  |

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