Note 6 - 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, 2018	Dec. 31, 2017
Options Outstanding, Beginning Balance (in shares)		2,681,072
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 7.18
Options Outstanding, Weighted-average Remaining Contractual Term (Year)		7 years 328 days	6 years 292 days
Options Outstanding, Aggregate Intrinsic Value		$ 614	$ 121
Options Granted (in shares)		1,891,600
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 2.65
Options Exercised (in shares)
Options Exercised, Weighted Average Exercise Price (in dollars per share)
Options Forfeited (in shares)		(83,433)
Options Forfeited, Weighted Average Exercise Price (in dollars per share)		$ 5.82
Options Expired (in shares)		(42,013)
Options Expired, Weighted Average Exercise Price (in dollars per share)		$ 13.30
Options Outstanding, Ending Balance (in shares)		4,447,226	2,681,072
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 5.22	$ 7.18
Options Exercisable (in shares)		1,844,871
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 7.44
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		6 years 36 days
Options Exercisable, Aggregate Intrinsic Value		$ 286
Options Vested or Expected to Vest (in shares)	[1]	4,447,226
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 5.22
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	7 years 328 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 614

[1] Represents the number of vested options at June 30, 2018 plus the number of unvested options expected to vest based on the unvested options outstanding at June 30, 2018.