Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Aug. 02, 2017	Nov. 12, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2017
Preferred Stock, Par or Stated Value Per Share			$ 0.001	$ 0.001
Number of Operating Segments			1
Number of Reportable Segments			1
Deferred Tax Assets, Operating Loss Carryforwards			$ 2,651	$ 42,156	$ 34,488
Available-for-sale Securities, Gross Realized Gain (Loss)			0	0	0
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities			0	0	0
Liability for Uncertainty in Income Taxes, Current			0	0
Deferred Tax Assets, Valuation Allowance			$ 6,949	$ 65,956	$ 52,234
Accounting Standards Update 2016-09 [Member]
Deferred Tax Assets, Operating Loss Carryforwards						$ 600
Deferred Tax Assets, Valuation Allowance						$ 600
Series A Convertible Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues	22,000
Payments of Stock Issuance Costs	$ 500
Preferred Stock, Par or Stated Value Per Share	$ 0.001		$ 0.001	$ 0.001
Shares Issued, Price Per Share	$ 1,000
Proceeds from Issuance of Convertible Preferred Stock	$ 22,000
Common Stock [Member]
Stock Issued During Period, Shares, New Issues			896,811
Sales Agreement [Member] | Cowen and Company, LLC [Member] | Common Stock [Member]
Sale of Stock, Maximum Value of Stock Offered		$ 40,000
Sale of Stock, Commission to Sales Agent, Percentage			3.00%
Stock Issued During Period, Shares, New Issues			896,811
Proceeds from Issuance of Common Stock			$ 1,400
Payments of Stock Issuance Costs			$ 100