Note 11 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Options available for grant (in shares)	1,601,202
Number of options outstanding (in shares)	6,521,146
Weighted average exercise price per share (in dollars per share)	$ 1.19
Weighted average remaining contractual life (Year)	7 years 109 days	6 years 328 days
Additional shares reserved (in shares)	1,918,868
Options available for grant, options granted (in shares)	(1,802,704)
Number of options outstanding, options granted (in shares)	1,802,704
Weighted average exercise price per share, options granted (in dollars per share)	$ 1.32
Options available for grant, options forfeited/cancelled/repurchase (in shares)	171,057
Number of options outstanding, options forfeited/cancelled/repurchase (in shares)	(171,057)
Weighted average exercise price per share, options forfeited/cancelled/repurchase (in dollars per share)	$ 2.35
Options available for grant (in shares)	1,888,423	1,601,202
Number of options outstanding (in shares)	8,152,793	6,521,146
Weighted average exercise price per share (in dollars per share)	$ 1.20	$ 1.19
Number of options outstanding, vested or expected to vest (in shares)	7,734,708
Weighted average exercise price per share, vested or expected to vest (in dollars per share)	$ 1.17
Weighted average remaining contractual life, vested or expected to vest (Year)	7 years 73 days
Number of options outstanding, exercisable (in shares)	4,490,966
Weighted average exercise price per share, exercisable (in dollars per share)	$ 0.81
Weighted average remaining contractual life, exercisable (Year)	5 years 219 days