Note 1 - Nature of Business and Financial Condition (Details Textual) $ in Thousands		1 Months Ended		2 Months Ended	12 Months Ended
	Jun. 01, 2018	Aug. 31, 2019 USD ($)	Feb. 28, 2018 USD ($)	Feb. 29, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Net Income (Loss) Attributable to Parent, Total					$ (28,660)	$ (27,976)
Retained Earnings (Accumulated Deficit), Ending Balance					(457,986)	(429,326)
Cash and Cash Equivalents, at Carrying Value, Ending Balance					16,302	33,734
Proceeds from Issuance of Common Stock					$ 11,589	$ 39,394
Reverse Stock Split [Member]
Stockholders' Equity Note, Stock Split, Conversion Ratio	20
At-the-market Offering [Member]
Increase (Decrease) in At-the-Market Offering of Stock		$ 10,700	$ 84,900
Stock Issued During Period, Shares, New Issues | shares					3,965,688	6,936,930
Proceeds from Issuance of Common Stock					$ 11,500	$ 38,900
Offering of Stock, Remaining Capacity, Values					$ 8,800
At-the-market Offering [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues | shares				425,766
Proceeds from Issuance of Common Stock				$ 900
Offering of Stock, Remaining Capacity, Shares | shares				7,800,000