Note 6 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Exercised, number of options (in shares)	(68,284)	(260,481)
Common Stock Options [Member]
Outstanding, number of options (in shares)	600,571
Outstanding, weighted average exercise price (in dollars per share)	$ 25.31
Outstanding, weighted average remaining contractual term (Year)	5 years 9 months 14 days		5 years 6 months 29 days
Granted, number of options (in shares)	51,002	48,934
Granted, weighted average exercise price (in dollars per share)	$ 42.92
Exercised, number of options (in shares)	(68,284)
Exercised, weighted average exercise price (in dollars per share)	$ 13.34
Forfeited, number of options (in shares)	(53,763)
Forfeited, weighted average exercise price (in dollars per share)	$ 40.48
Outstanding, number of options (in shares)	529,526		600,571
Outstanding, weighted average exercise price (in dollars per share)	$ 27.01		$ 25.31
Exercisable, number of options (in shares)	291,968
Weighted average exercise price (in dollars per share)	$ 18.24
Exercisable, weighted average remaining contractual term (Year)	4 years 4 months 9 days
Common Stock Options [Member] | Common Stock [Member]
Outstanding, aggregate intrinsic value	$ 11,203		$ 11,665
Exercised, aggregate intrinsic value	2,122
Exercisable, aggregate intrinsic value	$ 8,398