Note 1 - Organization and Basis of Presentation (Details Textual) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Oct. 13, 2020 USD ($)	Mar. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Mar. 31, 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		6,654,367	692,651	7,347,018
Proceeds from Issuance of Common Stock		$ 68.9	$ 5.5	$ 74.4
Proceeds from Issuance of Common Stock, Net		$ 65.7	$ 4.9	$ 70.6
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