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Note 18 - Variable Interest Entities - Business Trusts (Details) - USD ($) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 15, 2021 | Dec. 31, 2024 | Dec. 31, 2023 | Jan. 26, 2016
Preferred securities outstanding |  | $ 48.3 | $ 47.6 |
Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration] | Secured Overnight Financing Rate (SOFR) [Member] | Secured Overnight Financing Rate (SOFR) [Member] |  |
Mercantile Bank Capital Trust I [Member] |  |  |  |
Preferred securities outstanding |  | $ 21.0 |  | $ 32.0
Interest rate margin |  | 2.18% |  |
Firstbank Capital Trust I [Member] |  |  |  |
Preferred securities outstanding |  | $ 10.0 |  |
Interest rate margin |  | 1.99% |  |
Firstbank Capital Trust II [Member] |  |  |  |
Preferred securities outstanding |  | $ 10.0 |  |
Interest rate margin |  | 1.27% |  |
Firstbank Capital Trust III [Member] |  |  |  |
Preferred securities outstanding |  | $ 7.5 |  |
Interest rate margin |  | 1.35% |  |
Firstbank Capital Trust IV [Member] |  |  |  |
Preferred securities outstanding |  | $ 7.5 |  |
Interest rate margin |  | 1.35% |  |

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- Definition Trust preferred securities possesses characteristics of both equity and debt issues and are generally issued by bank holding companies. + References No definition available. + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Indicates type of variable interest rate on debt instrument. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 470 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477734/942-470-50-3 + Details
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- Details
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- Details
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- Details
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