Note 13 - Share-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Stock options outstanding, beginning of period (in shares)	3,491,937
Balance, weighted average exercise price, beginning of period (in dollars per share)	$ 1.78
Stock options outstanding, weighted average remaining contractual term (Year)	8 years 36 days	8 years 255 days
Stock options outstanding, aggregate intrinsic value	$ 1,890	$ 39,046
Stock options granted (in shares)	842,653
Stock options granted, weighted average exercise price (in dollars per share)	$ 7.66
Stock options exercised (in shares)	(87,542)
Stock options exercised, weighted average exercise price (in dollars per share)	$ 1.87
Stock options cancelled (in shares)	(33,948)
Stock options cancelled, weighted average exercise price (in dollars per share)	$ 0.48
Stock options outstanding, end of period (in shares)	4,213,100	3,491,937
Balance, weighted average exercise price, end of period (in dollars per share)	$ 2.96	$ 1.78
Exercisable, options outstanding (in shares)	1,556,234
Exercisable, weighted average exercise price (in dollars per share)	$ 2.28
Stock options exercisable, weighted average remaining contractual term (Year)	6 years 328 days
Stock options exercisable, aggregate intrinsic value	$ 844
Unexercisable, options outstanding (in shares)	2,656,866
Unexercisable, weighted average exercise price (in dollars per share)	$ 3.36
Stock options unexercisable, weighted average remaining contractual term (Year)	8 years 292 days
Stock options unexercisable, aggregate intrinsic value	$ 1,046