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Note 3 - Securities (Details Textual) | 12 Months Ended
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Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | 107 |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Total | $ 117,573,000 | $ 278,331,000 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total | 1,254,000 | 11,357,000 |
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 | $ 7,619,000
Debt Securities, Available-for-sale, Amortized Cost, Total | 329,979,000 | 347,795,000 |
Debt Securities, Available-for-sale, Total | 334,655,000 | 337,366,000 |
Collateralized Mortgage Backed Securities [Member] |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Total | 13,261,000 | 35,222,000 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total | 82,000 | 792,000 |
Proceeds from Sale of Available-for-sale Securities, Total | 0 | 0 | 5,000,000
Debt Securities, Available-for-sale, Amortized Cost, Total | 41,998,000 | 44,263,000 |
Debt Securities, Available-for-sale, Total | 42,470,000 | 43,658,000 |
Municipal General Obligation Bonds [Member] |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Total | 4,749,000 | 72,884,000 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total | 30,000 | 1,165,000 |
Proceeds from Sale of Available-for-sale Securities, Total | 0 | 0 | $ 2,600,000
Debt Securities, Available-for-sale, Amortized Cost, Total | 98,245,000 | 103,235,000 |
Debt Securities, Available-for-sale, Total | 101,079,000 | 102,497,000 |
State of Michigan and All Its Political Subdivisions [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 96,500,000 | 98,200,000 |
Debt Securities, Available-for-sale, Total | 99,400,000 | 97,400,000 |
All Other States and Their Political Subdivisions [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 5,900,000 | 8,700,000 |
Debt Securities, Available-for-sale, Total | 5,900,000 | 8,700,000 |
US Government Agency Debt Obligations and Mortgage Backed Securities [Member] |  |  |
Debt Securities, Available-for-sale, Restricted | $ 103,000,000 | $ 104,000,000 |

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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=118939692&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=118942338&loc=SL82898722-210454 + Details
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