Note 10 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options Outstanding, Beginning Balance (in shares)		2,166,254
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 8.55
Options Outstanding, Weighted-average Remaining Contractual Term, Beginning Balance (Year)		6 years 292 days	7 years 36 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 121	$ 124
Options Granted (in shares)		719,337	132,495	1,010,256
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 1.99	$ 7.11
Options Exercised (in shares)		(74,001)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.45
Options Forfeited (in shares)		(30,272)
Options Forfeited, Weighted Average Exercise Price (in dollars per share)		$ 10.69
Options Expired (in shares)		(100,246)
Options Expired, Weighted Average Exercise Price (in dollars per share)		$ 2.60
Options Outstanding, Ending Balance (in shares)		2,681,072	2,166,254
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 7.18	$ 8.55
Options Exercisable (in shares)		1,495,789
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 8.18
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		5 years 328 days
Options Exercisable, Aggregate Intrinsic Value		$ 85
Options Vested or Expected to Vest (in shares)	[1]	2,681,072
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 7.18
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	6 years 292 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 121

[1] Represents the number of vested options at December 31, 2017 plus the number of unvested options expected to vest based on the unvested options outstanding at December 31, 2017.