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		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Exercised, number of options (in shares)	(148,284)	(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)			6 years 2 months 1 day		5 years 6 months 29 days
Granted, number of options (in shares)			101,091	70,471
Granted, weighted average exercise price (in dollars per share)			$ 44.96
Exercised, number of options (in shares)			(68,284)
Exercised, weighted average exercise price (in dollars per share)			$ 13.34
Expired, number of options (in shares)			(22,837)
Expired, weighted average exercise price (in dollars per share)			$ 9.74
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)			556,778		600,571
Outstanding, weighted average exercise price (in dollars per share)			$ 29.52		$ 25.31
Exercisable, number of options (in shares)			290,668
Weighted average exercise price (in dollars per share)			$ 21.57
Exercisable, weighted average remaining contractual term (Year)			4 years 9 months 18 days
Common Stock Options [Member] | Common Stock [Member]
Outstanding, aggregate intrinsic value			$ 9,942		$ 11,665
Exercised, aggregate intrinsic value			2,122
Exercisable, aggregate intrinsic value			$ 7,314