Note 10 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Oct. 23, 2017	Oct. 11, 2010	Jan. 05, 2009	May 16, 2008	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Mar. 29, 2017	Sep. 21, 2012	Sep. 29, 2009	Jan. 08, 2009
Bonus and Royalty Accrued					$ 0
Stock Compensation Expense Related to Royalties and Stock Bonuses Accrued But Not Issued							$ 3
Shares Issued, Value, Share-based Payment Arrangement, before Forfeiture					179	[1]	372	[2],[3]	$ 19,360	[4]
Sale of Equity, Registration, Maximum Value												$ 100,000
Stock Issued During Period, Shares, New Issues	7,392,856
Shares Issued, Price Per Share	$ 2.80
Proceeds from Issuance or Sale of Equity, Total	$ 18,900								$ 18,905
Common Stock, Par or Stated Value Per Share					$ 0.001		$ 0.001
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross					0		0		0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant					0
2008 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized				1,500,000										3,000,000
2008 Equity Incentive Plan [Member] | Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent				110.00%
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period				10 years
2009 Equity Incentive Plan (Executive) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized													330,000		4,200,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period			10 years
Common Stock, Par or Stated Value Per Share															$ 0.001
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		3,060,382
Over-Allotment Option [Member]
Stock Issued During Period, Shares, New Issues	964,285
Cash Less Warrant [Member]
Proceeds from Issuance of Common Stock					$ 0
Amortized From Prepaid Expense [Member]
Shares Issued, Value, Share-based Payment Arrangement, before Forfeiture							$ 24
Included in Prepaid Expense [Member]
Shares Issued, Value, Share-based Payment Arrangement, before Forfeiture									$ 24
Included in Accrued Liabilities [Member]
Share-based Payment Arrangement, Noncash Expense, Total									170
Other Expense [Member]
Bonus and Royalty Accrued and Expensed					$ 36		$ 3		$ 170		$ 176

[1] Total excludes $36 thousand of 2019 stock compensation and $3 thousand of 2018 stock compensation related to royalties that were recorded as expense but not issued and outstanding as of December 31, 2019.

[2] Excludes $24 thousand of stock compensation for stock issued in 2017 that was amortized from prepaid expense in 2018.

[3] Total excludes $3 thousand of stock compensation related to royalties that were recorded as expense but not issued and outstanding as of December 31, 2018.

[4] Does not include stock expense of $170 thousand included in accrued liabilities at December 31, 2017 for unissued stock.