Note 1 - The Company and Basis of Presentation (Details Textual) - USD ($)				3 Months Ended	6 Months Ended
	Feb. 12, 2018	Nov. 08, 2017	Mar. 22, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Proceeds from Issuance of Common Stock					$ 33,396,000	$ 31,440,000
February 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues	11,500,000
Share Price	$ 3
Proceeds from Issuance of Common Stock	$ 34,500,000
Proceeds From Issuance of Common Stock, Net	$ 32,213,000
November 2017 ATM Facility [Member]
Stock Issued During Period, Shares, New Issues					332,778
Proceeds From Issuance of Common Stock, Net					$ 1,308,000
Aggregate Offering Price		$ 25,000,000
ATM Facility [Member]
Stock Issued During Period, Shares, New Issues				65,252	273,529
Proceeds From Issuance of Common Stock, Net				$ 172,000	$ 1,183,000
March 2017 Offering [Member]
Stock Issued During Period, Shares, New Issues			8,625,000
Share Price			$ 4
Proceeds from Issuance of Common Stock			$ 34,500,000
Proceeds From Issuance of Common Stock, Net			$ 31,440,000