Note 1 - Organization and Basis of Presentation (Details Textual) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended	9 Months Ended
	Oct. 13, 2020 USD ($)	Mar. 31, 2021	Dec. 31, 2020 USD ($) $ / shares shares	Jun. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2021 USD ($) $ / shares shares	Jun. 08, 2020 $ / shares shares	Jun. 07, 2020 shares	Apr. 11, 2019 $ / shares
Common Stock, Par or Stated Value Per Share (in dollars per share) | $ / shares			$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Common Stock, Shares Authorized (in shares) | shares			150,000,000	150,000,000	150,000,000	150,000,000	100,000,000
Number of Operating Segments		1
Open Market Sale Agreement [Member]
Open Market Sale Agreement, Maximum Aggregate Offering Price	$ 250.0
Payments of Stock Issuance Costs	$ 0.3
Open Market Sale Agreement, Sales Commission, Percentage	4.50%
Stock Issued During Period, Shares, New Issues (in shares) | shares			692,651	10,958,908	11,651,559
Proceeds from Issuance of Common Stock			$ 5.5	$ 106.9	$ 112.4
Proceeds from Issuance of Common Stock, Net			$ 4.9	$ 101.9	$ 106.8
Warrants in Connection With the April 2019 Offering [Member]
Pre-funded Warrant, Accompanying Warrant, Amount Paid Per Share (in dollars per share) | $ / shares								$ 0.10