Note 7 - Share-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Stock options outstanding, beginning of period (in shares) | shares	4,213,100
Balance, weighted average exercise price, beginning of period (in dollars per share) | $ / shares	$ 2.96
Stock options granted (in shares) | shares	1,429,075
Stock options granted, weighted average exercise price (in dollars per share) | $ / shares	$ 2.60
Stock options exercised (in shares) | shares	(2,488)
Stock options exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.80
Stock options cancelled (in shares) | shares	(20,833)
Stock options cancelled, weighted average exercise price (in dollars per share) | $ / shares	$ 7.56
Stock options outstanding, end of period (in shares) | shares	5,618,854
Balance, weighted average exercise price, end of period (in dollars per share) | $ / shares	$ 2.85