Note 7 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Jun. 01, 2020	Nov. 21, 2019	Nov. 18, 2019	Aug. 06, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 24, 2020	Nov. 08, 2017
Maximum Aggregate Offering Price								$ 500,000	$ 200,000
Stock Issued During Period, Shares, New Issues (in shares)	22,044		43,621,000	10,455,000
Shares Issued, Price Per Share (in dollars per share)	$ 4.50	$ 1.69	$ 1.45	$ 5.50
Proceeds from Issuance of Common Stock			$ 58,500	$ 53,400	$ 93,279	$ 58,500	$ 53,400
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		11,765,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 0.01
Proceeds from Issuance of Warrants		$ 19,882			$ 14,817	$ 19,882
Proceeds from Issuance or Sale of Equity, Total	$ 108,096,000
Warrants Issued in Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	3,511
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 0.01
Class of Warrant or Right, Warrants Issued, Price Per Warrant (in dollars per share)	$ 4.49