Note 3 - Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Total	$ 24,394	$ 32,633
Cash Equivalents [Member]
Cash equivalents	10,425	11,662
US Government Agencies Debt Securities [Member]
Government securities	13,969	20,971
Fair Value, Inputs, Level 1 [Member]
Total	24,394	32,633
Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member]
Cash equivalents	10,425	11,662
Fair Value, Inputs, Level 1 [Member] \| US Government Agencies Debt Securities [Member]
Government securities	13,969	20,971
Fair Value, Inputs, Level 2 [Member]
Total
Fair Value, Inputs, Level 2 [Member] \| Cash Equivalents [Member]
Cash equivalents
Fair Value, Inputs, Level 2 [Member] \| US Government Agencies Debt Securities [Member]
Government securities
Fair Value, Inputs, Level 3 [Member]
Total
Fair Value, Inputs, Level 3 [Member] \| Cash Equivalents [Member]
Cash equivalents
Fair Value, Inputs, Level 3 [Member] \| US Government Agencies Debt Securities [Member]
Government securities

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes 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.

+ References

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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