Note 9 - Summary of Stock Options - Summary of Option Activity Under All Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Options outstanding, Shares (in shares)	2,694,224
Options outstanding, Weighted average exercise price (in dollars per share)	$ 5.34	$ 5.80
Options outstanding, Weighted Average Remaining Contractual Term (Year)	8 years 229 days	8 years 211 days
Options outstanding, Aggregate Intrinsic Value	$ 12,750	$ 249,154
Options granted, Shares (in shares)	1,253,559
Options granted, Weighted average exercise price (in dollars per share)	$ 4.40
Options exercised, Shares (in shares)
Options exercised, Weighted average exercise price (in dollars per share)
Options canceled (in shares)	(256,881)
Options canceled, Weighted average exercise price (in dollars per share)	$ 5.66
Options outstanding, Shares (in shares)	3,690,902	2,694,224
Vested and exercisable and expected to vest, end of period, Number Of Shares (in shares)	3,445,833
Vested and exercisable and expected to vest, end of period, Weighted Average Exercise Price (in dollars per share)	$ 5.36
Vested and exercisable and expected to vest, end of period, Weighted Average Remaining Contractual Term (Year)	8 years 208 days
Vested and exercisable and expected to vest, end of period, Aggregate Intrinsic Value	$ 12,628
Vested and exercisable, end of period, Number Of Shares (in shares)	1,006,199
Vested and exercisable, end of period, Weighted Average Exercise Price (in dollars per share)	$ 5.74
Vested and exercisable, end of period, Weighted Average Remaining Contractual Term (Year)	6 years 350 days
Vested and exercisable, end of period, Aggregate Intrinsic Value	$ 9,297