Note 11 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jun. 02, 2020	Dec. 31, 2021	Dec. 31, 2020
Common Stock, Shares Authorized (in shares)		150,000,000	150,000,000
Common Stock, Par or Stated Value Per Share (in dollars per share)		$ 0.0001	$ 0.0001
Preferred Stock, Shares Authorized (in shares)		5,000,000	5,000,000
Preferred Stock, Par or Stated Value Per Share (in dollars per share)		$ 0.0001	$ 0.0001
Proceeds from issuance of Common Stock and Warrants		$ 0	$ 12,400
Proceeds from Warrant Exercises		$ 12,047	$ 26,191
Class of Warrant or Right, Exercised During Period (in shares)		1,815,012,000	5,155,976
Payments of Facilitating Exercise of Warrants			$ 1,017
Proceeds from Warrant Exercises, Net of Exercise Costs			25,174
Officer [Member]
Proceeds from Warrant Exercises			$ 1,667
Class of Warrant or Right, Exercised During Period (in shares)			333,334
Class of Warrant or Right, Exercised During Period, Exercise Price (in dollars per share)			$ 5.00
Warrants Issued with June 2019 Follow-On Offering [Member]
Warrants and Rights Outstanding, Term (Year)	2 years
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 6.00
June 2020 Follow-on Public Offering [Member]
Number of Shares Per Unit Issued (in shares)	1
Number of Warrants Per Unit (in shares)	1
Shares Issued, Price Per Share (in dollars per share)	$ 6.00
Equity Offering, Units Issued (in shares)	2,066,667
Proceeds from issuance of Common Stock and Warrants	$ 12,400
Proceeds From Issuance of Common Stock and Warrants, Net of Issuance Costs	$ 11,670