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Note 3 - Fair Value Measurements (Details Textual) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Cash Equivalents, at Carrying Value, Total | $ 9,100,000 | $ 49,400,000 |
Available-for-sale Securities, Current, Total | 300,535,000 | 27,796,000 |
Debt Securities, Available-for-sale, Realized Gain (Loss), Total | 77,000 | 7,000 | $ 23,000
Available-for-sale Securities, Gross Realized Gains | 0 | 0 | 0
Available-for-sale Securities, Gross Realized Losses | $ 0 | $ 0 | $ 0

X
- Definition Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer. + References No definition available. + Details
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Period Type: | instant

X
- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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X
- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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X
- Definition Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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X
- Definition Amount of realized gain (loss) on 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/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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