Note 5 - Share-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Outstanding at beginning of year (in shares) | shares	2,390,012
Outstanding at beginning of year, weighted average exercise price (in dollars per share) | $ / shares	$ 1.85
Granted (in shares) | shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised (in shares) | shares	(62,438)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.16
Forfeited (in shares) | shares	(730)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 2.62
Expired (in shares) | shares
Expired, weighted average exercise price (in dollars per share) | $ / shares
Outstanding at March 31, 2018 (in shares) | shares	2,326,844
Outstanding at end of year, weighted average exercise price (in dollars per share) | $ / shares	$ 1.87
Stock options exercisable at March 31, 2018 (in shares) | shares	1,637,951
Stock options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.76