Note 5 - Share-based Compensation - Stock Option Activity (Details) - Share-based Payment Arrangement, Option [Member]	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Outstanding (in shares)	2,043,402
Outstanding at beginning of year, weighted average exercise price (in dollars per share) | $ / shares	$ 1.91
Granted (in shares)
Exercised (in shares)	(99,697)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.61
Forfeited (in shares)	(3,438)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 5.69
Expired (in shares)
Expired, weighted average exercise price (in dollars per share) | $ / shares
Outstanding (in shares)	1,940,267
Outstanding at end of year, weighted average exercise price (in dollars per share) | $ / shares	$ 1.92
Stock options exercisable (in shares)	1,657,436
Stock options exercisable at year end, weighted average exercise price (in dollars per share) | $ / shares	$ 1.89