Note 1 - Organization and Operations (Details Textual) - USD ($)			3 Months Ended	9 Months Ended
	Aug. 02, 2017	Nov. 12, 2015	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Cash, Cash Equivalents, and Short-term Investments			$ 47,400,000	$ 47,400,000
Retained Earnings (Accumulated Deficit)			$ (184,190,000)	(184,190,000)		$ (159,777,000)
Proceeds from Issuance of Common Stock				$ 1,287,000
Preferred Stock, Par or Stated Value Per Share			$ 0.001	$ 0.001		$ 0.001
Proceeds from Issuance of Convertible Preferred Stock				$ 21,536,000
Series A Convertible Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues	22,000
Payments of Stock Issuance Costs	$ 500,000
Preferred Stock, Par or Stated Value Per Share	$ 0.001		$ 0.001	$ 0.001		$ 0.001
Shares Issued, Price Per Share	$ 1,000
Proceeds from Issuance of Convertible Preferred Stock	$ 22,000,000
Sales Agreement [Member] | Cowen and Company, LLC [Member]
Sale of Stock, Maximum Value of Stock Offered		$ 40,000,000
Sales of Stock, Commission Paid				3.00%
Stock Issued During Period, Shares, New Issues			0	896,811
Proceeds from Issuance of Common Stock			$ 0	$ 1,400,000
Payments of Stock Issuance Costs			$ 0	$ 200,000