Note 1 - The Company and Basis of Presentation (Details Textual) - USD ($)				1 Months Ended				3 Months Ended		9 Months Ended
	Aug. 16, 2019	Feb. 12, 2018	Nov. 08, 2017	Aug. 31, 2019	Dec. 31, 2018		Feb. 28, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017
Proceeds from Issuance of Common Stock										$ 6,322,000	$ 33,407,000
Retained Earnings (Accumulated Deficit), Ending Balance					$ (155,385,000)	[1]		$ (183,094,000)		(183,094,000)
Cash and Cash Equivalents, at Carrying Value, Ending Balance					$ 29,523,000	[1]		9,086,000	$ 20,097,000	9,086,000	20,097,000	$ 20,730,000
Working Capital								12,836,000		12,836,000
Net Cash Provided by (Used in) Operating Activities, Total								$ (6,608,000)		$ (25,862,000)	$ (33,245,000)
August 2019 ATM Facility [Member]
Aggregate Offering Price	$ 6,760,000			$ 6,760,000
Stock Issued During Period, Shares, New Issues								1,004,171
Proceeds From Issuance of Common Stock, Net								$ 6,322,000
November 2017 ATM Facility [Member]
Aggregate Offering Price			$ 25,000,000	$ 25,000,000
Stock Issued During Period, Shares, New Issues				3,364				0	37	0	2,771
Proceeds From Issuance of Common Stock, Net				$ 1,318,000				$ 11,000	$ 11,000	$ 1,194,000	$ 1,194,000
December 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues					147,285
Proceeds From Issuance of Common Stock, Net					$ 20,385,000
Proceeds from Issuance of Common Stock					$ 22,093,000
February 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues							115,000
Proceeds From Issuance of Common Stock, Net		$ 32,214,000					$ 32,214,000
Proceeds from Issuance of Common Stock		$ 34,500,000					$ 34,500,000

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