Note 2 - Summary of Significant Accounting Policies - Outstanding Stock Options and Stock Warrants Excluded From the Diluted Net Loss Per Share Computation (Details) - shares shares in Thousands	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020
Anti-dilutive securities (in shares)	10,955	9,797
Share-based Payment Arrangement, Option [Member]
Anti-dilutive securities (in shares)	3,873	2,692
Warrant [Member]
Anti-dilutive securities (in shares)	7,082	7,105

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=124432515&loc=d3e3550-109257

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration