Note 9 - Summary of Stock Options - Summary of Option Activity Under All Plans (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Options outstanding, Shares (in shares) | shares	2,694,224
Options outstanding, Weighted average exercise price (in dollars per share) | $ / shares	$ 5.80
Options outstanding, Weighted Average Remaining Contractual Term (Year)	8 years 292 days	8 years 219 days
Options outstanding, Aggregate Intrinsic Value | $	$ 433,626	$ 249,154
Options granted, Shares (in shares) | shares	2,023,559
Options granted, Weighted average exercise price (in dollars per share) | $ / shares	$ 3.60
Options exercised, Shares (in shares) | shares
Options exercised, Weighted average exercise price (in dollars per share) | $ / shares
Options canceled, Shares (in shares) | shares	(508,428)
Options canceled, Weighted average exercise price (in dollars per share) | $ / shares	$ 5.66
Options outstanding, Shares (in shares) | shares	4,209,355	2,694,224
Options outstanding, Weighted average exercise price (in dollars per share) | $ / shares	$ 4.76	$ 5.80
Vested and exercisable and expected to vest, end of period, Number Of Shares (in shares) | shares	3,935,007
Vested and exercisable and expected to vest, end of period, Weighted Average Exercise Price (in dollars per share) | $ / shares	$ 4.80
Vested and exercisable and expected to vest, end of period, Weighted Average Remaining Contractual Term (Year)	8 years 255 days
Vested and exercisable and expected to vest, end of period, Aggregate Intrinsic Value | $	$ 392,290
Vested and exercisable, end of period, Number Of Shares (in shares) | shares	1,124,400
Vested and exercisable, end of period, Weighted Average Exercise Price (in dollars per share) | $ / shares	$ 5.65
Vested and exercisable, end of period, Weighted Average Remaining Contractual Term (Year)	7 years 182 days
Vested and exercisable, end of period, Aggregate Intrinsic Value | $	$ 10,167