Note 1 - Organization and Operations (Details Textual) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	6 Months Ended
	Aug. 02, 2017	Nov. 12, 2015	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Cash, Cash Equivalents, and Short-term Investments			$ 31,700	$ 31,700
Retained Earnings (Accumulated Deficit)			$ (171,883)	(171,883)		$ (159,777)
Proceeds from Issuance of Common Stock				$ 1,287
Preferred Stock, Par or Stated Value Per Share			$ 0.001	$ 0.001		$ 0.001
Sales Agreement [Member] | Cowen and Company, LLC [Member]
Sale of Stock, Maximum Value of Stock Offered		$ 40,000
Sales of Stock, Commission Paid				3.00%
Payments of Stock Issuance Costs			$ 100	$ 200
Stock Issued During Period, Shares, New Issues			803,299	896,811
Proceeds from Issuance of Common Stock			$ 1,300	$ 1,400
Subsequent Event [Member] | Series A Convertible Preferred Stock [Member]
Proceeds from Issuance or Sale of Equity	$ 22,000
Stock Issued During Period, Shares, New Issues	22,000
Preferred Stock, Par or Stated Value Per Share	$ 0.001
Shares Issued, Price Per Share	$ 1,000
Proceeds from Issuance of Convertible Preferred Stock	$ 22,000