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Note 2 - Securities (Details Textual) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Total | $ 296,119,000 | $ 252,521,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total | 14,039,000 | 7,559,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance | 0 |
Debt Securities, Available-for-sale, Amortized Cost, Total | 350,335,000 | 341,948,000
Debt Securities, Available-for-sale, Total | 336,988,000 | 335,744,000
State of Michigan and All Its Political Subdivisions [Member] |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 112,000,000 | 112,000,000
Debt Securities, Available-for-sale, Total | 111,000,000 | 112,000,000
All Other States and Their Political Subdivisions [Member] |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 10,800,000 | 12,900,000
Debt Securities, Available-for-sale, Total | $ 10,700,000 | 13,000,000
Securities Other Than US Government Agencies and the State of Michigan and Political Subdivisions [Member] | Stockholders' Equity, Total [Member] | Securities Issued Concentration Risk [Member] |  |
Concentration Risk, Percentage | 10.00% |
US Government Agency Debt Obligations and Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale, Restricted | $ 105,000,000 | $ 119,000,000
Debt Securities [Member] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | 394 |
Mutual Funds [Member] |  |
Equity Securities, Number of Positions | 1 |

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