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Note 3 - Securities (Details Textual) | 12 Months Ended
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Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $ 253,210,000 | $ 163,876,000 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 9,429,000 | 2,069,000 |
Proceeds from Sale of Available-for-sale Securities | 264,000 | 1,483,000 | $ 0
Available-for-sale Debt Securities, Amortized Cost Basis | 336,562,000 |  |
Available-for-sale Securities | $ 328,060,000 | 346,992,000 |
Debt Securities [Member] |  |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | 441 |  |
Mutual Funds [Member] |  |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | 1 |  |
Municipal General Obligation Bonds [Member] |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $ 93,629,000 | 32,115,000 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 1,549,000 | 205,000 |
Proceeds from Sale of Available-for-sale Securities | 300,000 | 1,500,000 | $ 0
Available-for-sale Securities | 119,047,000 | 122,023,000 |
State of Michigan and All Its Political Subdivisions [Member] |  |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 109,000,000 | 106,000,000 |
Available-for-sale Securities | 107,000,000 | 107,000,000 |
All Other States and Their Political Subdivisions [Member] |  |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 19,500,000 | 24,000,000 |
Available-for-sale Securities | $ 19,500,000 | 24,100,000 |
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] |  |  |
Available-for-sale Securities, Restricted | $ 132,000,000 | $ 155,000,000 |

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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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