Note 6 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Outstanding, number of options (in shares) | shares	1,373,209
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 15.99
Granted, number of options (in shares) | shares	73,331
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 38.30
Exercised, number of options (in shares) | shares	(86,247)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 7.34
Exercised, aggregate intrinsic value | $	$ 2,717
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	1,360,293
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 17.74
Outstanding, weighted average remaining contractual term (Year)	5 years 80 days
Outstanding, aggregate intrinsic value | $	$ 28,379
Exercisable, number of options (in shares) | shares	907,432
Weighted average exercise price (in dollars per share) | $ / shares	$ 14.44
Exercisable, weighted average remaining contractual term (Year)	3 years 332 days
Exercisable, aggregate intrinsic value | $	$ 21,928