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Note 19 - Variable Interest Entities - Business Trusts (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Jan. 26, 2016
Preferred securities outstanding | $ 46,900,000 | $ 46,200,000 |
Mercantile Bank Capital Trust I [Member] |  |  |
Preferred securities outstanding | $ 21,000,000 |  | $ 32,000,000.0
Mercantile Bank Capital Trust I [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |
Interest rate margin | 2.18% |  |
Firstbank Capital Trust I [Member] |  |  |
Preferred securities outstanding | $ 10,000,000 |  |
Firstbank Capital Trust I [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |
Interest rate margin | 1.99% |  |
Firstbank Capital Trust II [Member] |  |  |
Preferred securities outstanding | $ 10,000,000 |  |
Firstbank Capital Trust II [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |
Interest rate margin | 1.27% |  |
Firstbank Capital Trust III [Member] |  |  |
Preferred securities outstanding | $ 7,500,000 |  |
Firstbank Capital Trust III [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |
Interest rate margin | 1.35% |  |
Firstbank Capital Trust IV [Member] |  |  |
Preferred securities outstanding | $ 7,500,000 |  |
Firstbank Capital Trust IV [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |
Interest rate margin | 1.35% |  |

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