Note 7 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2019	Dec. 31, 2018
Options Outstanding, Beginning Balance (in shares)		4,597,226
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 5.12
Options Outstanding, Weighted-average Remaining Contractual Term (Year)		6 years 109 days	7 years 146 days
Options Outstanding, Aggregate Intrinsic Value			$ 404
Options Granted (in shares)		1,182,500
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 2.66
Options Exercised (in shares)
Options Exercised, Weighted Average Exercise Price (in dollars per share)
Options Forfeited (in shares)	[1]	(1,514,779)
Options Forfeited, Weighted Average Exercise Price (in dollars per share)	[1]	$ 2.87
Options Expired (in shares)	[1]	(606,667)
Options Expired, Weighted Average Exercise Price (in dollars per share)	[1]	$ 5.14
Options Outstanding, Ending Balance (in shares)		3,658,280	4,597,226
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 5.25	$ 5.12
Options Exercisable (in shares)		2,184,564
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 6.91
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		4 years 219 days
Options Exercisable, Aggregate Intrinsic Value
Options Vested or Expected to Vest (in shares)	[2]	3,658,280
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[2]	$ 5.25
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[2]	6 years 109 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[2]

[1] Represents the number of options cancelled during the six months ended June 30, 2019 as a result of employees that were terminated due to the reduction in force.

[2] Represents the number of vested options at June 30, 2019 plus the number of unvested options expected to vest based on the unvested options outstanding at June 30, 2019.