Note 2 - Investment Securities (Details Textual) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Number of Securities	604
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Total	290
Aggregate Depreciation from Amortized Cost Basis, Percent	1.80%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	105
Other than Temporary Impairment Losses, Investments, Total	$ 0	$ 0
Proceeds from Sale and Maturity of Marketable Securities, Total	$ 0	$ 0
General Obligation Bonds [Member] \| Stockholders' Equity, Total [Member] \| Credit Concentration Risk [Member]
Concentration Risk, Percentage	0.00%		0.00%
Revenue Bonds [Member] \| Stockholders' Equity, Total [Member] \| Credit Concentration Risk [Member]
Concentration Risk, Percentage	0.00%		0.00%
Municipal Bonds [Member] \| General Obligation Bonds [Member]
Number of Issuers	128		131
Other Investments, Total	$ 101,300		$ 108,000
Number of States Investments Held in	26		26
Municipal Bonds [Member] \| General Obligation Bonds [Member] \| Minimum [Member]
Other Investments, Total	$ 5,000		$ 5,000
Municipal Bonds [Member] \| General Obligation Bonds [Member] \| Aggregate Fair Value Exceeding 5 Million [Member]
Number of States Investments Held in	6		6
Municipal Bonds [Member] \| Revenue Bonds [Member]
Number of Issuers	148		145
Other Investments, Total	$ 334,600		$ 337,300
Number of States Investments Held in	16		16
Municipal Bonds [Member] \| Revenue Bonds [Member] \| Minimum [Member]
Other Investments, Total	$ 5,000		$ 5,000
Municipal Bonds [Member] \| Revenue Bonds [Member] \| Aggregate Fair Value Exceeding 5 Million [Member]
Number of States Investments Held in	7		7

The aggregate depreciation from amortized cost basis as a percent.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Amount of investments classified as other.

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The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period.

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