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Note 2 - Securities (Details Textual) | 12 Months Ended
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Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | 64 |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Total | $ 119,073,000 | $ 117,573,000 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total | 899,000 | 1,254,000 |
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance | 0 |  |
Proceeds from Sale of Available-for-sale Securities, Total | 0 | 0 | $ 0
Debt Securities, Available-for-sale, Amortized Cost, Total | 380,403,000 | 329,979,000 |
Debt Securities, Available-for-sale, Total | 387,347,000 | 334,655,000 |
State of Michigan and All Its Political Subdivisions [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 109,000,000 | 96,500,000 |
Debt Securities, Available-for-sale, Total | 116,000,000 | 99,400,000 |
All Other States and Their Political Subdivisions [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 4,100,000 | 5,900,000 |
Debt Securities, Available-for-sale, Total | 4,200,000 | 5,900,000 |
US Government Agency Debt Obligations and Mortgage Backed Securities [Member] |  |  |
Debt Securities, Available-for-sale, Restricted | $ 118,000,000 | $ 103,000,000 |

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