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Note 20 - Subordinated Debentures - Business Trusts (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 | Jan. 26, 2016 | Dec. 31, 2015
Preferred securities outstanding | $ 42,800,000 |  | $ 53,100,000
Mercantile Bank Capital Trust I [Member] |  |  |
Preferred securities outstanding | $ 21,000,000 | $ 32,000,000 |
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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- 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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Balance Type: | credit
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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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- Details
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- Details
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- Details
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- Details
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