Note 8 - Stockholders' Equity - Reserved Shares of Common Stock for Issuance (Details) - shares	Jun. 30, 2020	Dec. 31, 2019
Stock options and restricted stock units issued and outstanding (in shares)	5,534,822	5,562,484
Total (in shares)	8,854,709	7,690,456
Share-based Payment Arrangement, Option [Member]
Common stock available for issuance (in shares)	3,207,661	1,954,804
Employee Stock Purchase Plan Shares [Member]
Common stock available for issuance (in shares)	112,226	173,168

Aggregate number of common shares reserved for future issuance.

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 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Number of options outstanding, including both vested and non-vested options.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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-Paragraph 2
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