Note 1 - Organization and Basis of Presentation (Details Textual) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 15, 2021 USD ($)	Oct. 13, 2020 USD ($)	Sep. 30, 2021 $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 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	$ 0.0001
Common Stock, Shares Authorized (in shares) | shares			150,000,000	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
Proceeds from Issuance of Common Stock				$ 5.5
Proceeds from Issuance of Common Stock, Net				$ 4.9
The October 2020 Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares					13,239,839	13,932,490
Proceeds from Issuance of Common Stock					$ 127.9	$ 133.4
Proceeds from Issuance of Common Stock, Net					$ 122.2	$ 127.1
The September 2021 ATM [Member]
Open Market Sale Agreement, Maximum Aggregate Offering Price	$ 100.0
Payments of Stock Issuance Costs	$ 0.2
Open Market Sale Agreement, Sales Commission, Percentage	3.00%
Stock Issued During Period, Shares, New Issues (in shares) | shares			0
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