Note 9 - Summary of Stock Options - Summary of Option Activity Under All Plans (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Options outstanding, Shares (in shares) | shares	1,909,764
Options outstanding, Weighted average exercise price (in dollars per share) | $ / shares	$ 6.19
Options outstanding, weighted average remaining contractual term (Year)	8 years 255 days	9 years 43 days
Options outstanding, Aggregate Intrinsic Value | $	$ 387,178	$ 211,396
Options granted, Shares (in shares) | shares	739,985
Options granted, Weighted average exercise price (in dollars per share) | $ / shares	$ 5.86
Options exercised, Shares (in shares) | shares	(7,730)
Options exercised, Weighted average exercise price (in dollars per share) | $ / shares	$ 4.02
Options canceled, Shares (in shares) | shares	(152,040)
Options canceled, Weighted average exercise price (in dollars per share) | $ / shares	$ 9.82
Options outstanding, Shares (in shares) | shares	2,489,979	1,909,764
Options outstanding, Weighted average exercise priceod (in dollars per share) | $ / shares	$ 5.88	$ 6.19
Vested and exercisable and expected to vest, end of period, Number Of Shares (in shares) | shares	2,337,350
Vested and exercisable and expected to vest, end of period, Weighted Average Exercise Price (in dollars per share) | $ / shares	$ 5.88
Vested and exercisable and expected to vest, end of period, Weighted Average Remaining Contractual Term (Year)	8 years 244 days
Vested and exercisable and expected to vest, end of period, Aggregate Intrinsic Value | $	$ 369,478
Vested and exercisable, end of period, Number Of Shares (in shares) | shares	734,721
Vested and exercisable, end of period, Weighted Average Exercise Price (in dollars per share) | $ / shares	$ 5.90
Vested and exercisable, end of period, Weighted Average Remaining Contractual Term (Year)	7 years 295 days
Vested and exercisable, end of period, Aggregate Intrinsic Value | $	$ 162,632