Note 1 - The Company and Basis of Presentation (Details Textual) - USD ($)			1 Months Ended			3 Months Ended		6 Months Ended		20 Months Ended
	Feb. 12, 2018	Nov. 08, 2017	Dec. 31, 2018		Feb. 28, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Jun. 08, 2018	Dec. 31, 2017
Shares Issued, Price Per Share											$ 0.91	$ 2.56
Proceeds from Issuance of Common Stock									$ 33,396,000
Retained Earnings (Accumulated Deficit), Ending Balance			$ (155,385,000)	[1]		$ (175,138,000)		(175,138,000)		$ (175,138,000)
Cash and Cash Equivalents, at Carrying Value, Ending Balance			$ 29,523,000	[1]		9,506,000	$ 30,213,000	9,506,000	30,213,000	9,506,000			$ 20,730,000
Working Capital						13,660,000		13,660,000		$ 13,660,000
Net Cash Provided by (Used in) Operating Activities, Total						$ (7,696,000)		$ (19,254,000)	$ (23,234,000)
December 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues			14,728,504
Shares Issued, Price Per Share			$ 1.50
Proceeds from Issuance of Common Stock			$ 22,093,000
Proceeds From Issuance of Common Stock, Net			$ 20,385,000
February 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues	11,500,000				11,500,000
Shares Issued, Price Per Share	$ 3				$ 3
Proceeds from Issuance of Common Stock	$ 34,500,000				$ 34,500,000
Proceeds From Issuance of Common Stock, Net	$ 32,214,000				$ 32,214,000
November 2017 ATM Facility [Member]
Stock Issued During Period, Shares, New Issues						0	65,252	0	273,529	336,498
Proceeds From Issuance of Common Stock, Net							$ 172,000		$ 1,183,000	$ 1,318,000
Aggregate Offering Price		$ 25,000,000

[1] The condensed consolidated balance sheet as of December 31, 2018 has been derived from the audited consolidated financial statements as of that date.