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Note 2 - Securities (Details Textual) | 12 Months Ended
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Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | 333 |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Total | $ 461,553,000 | $ 119,073,000 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total | 9,958,000 | 899,000 |
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance | 0 |  |
Proceeds from Sale of Debt Securities, Available-for-sale | 0 | 0 | $ 0
Debt Securities, Available-for-sale, Amortized Cost, Total | 597,463,000 | 380,403,000 |
Debt Securities, Available-for-sale, Total | 592,743,000 | 387,347,000 |
State of Michigan and All Its Political Subdivisions [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 155,000,000 | 109,000,000 |
Debt Securities, Available-for-sale, Total | 158,000,000 | 116,000,000 |
All Other States and Their Political Subdivisions [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 1,700,000 | 4,100,000 |
Debt Securities, Available-for-sale, Total | 1,700,000 | 4,200,000 |
US Government Agency Debt Obligations and Mortgage Backed Securities [Member] |  |  |
Debt Securities, Available-for-sale, Restricted | $ 197,000,000 | $ 118,000,000 |

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 + Details
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