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					9 Months Ended		12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Exercised, number of options (in shares)	(10,000)	(68,284)	(103,608)	(148,284)	(260,481)
Common Stock Options [Member]
Beginning balance, outstanding, number of options (in shares)		600,571				600,571
Beginning balance, outstanding, weighted average exercise price (in dollars per share)		$ 25.31				$ 25.31
Outstanding, weighted average remaining contractual term (Year)						5 years 11 months 4 days		5 years 6 months 29 days
Outstanding, aggregate intrinsic value	$ 7,697					$ 7,697		$ 11,665
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)						(78,284)
Exercised, weighted average exercise price (in dollars per share)						$ 13.45
Exercised, aggregate intrinsic value						$ 2,416
Expired, number of options (in shares)						(84,432)
Expired, weighted average exercise price (in dollars per share)						$ 38.38
Forfeited, number of options (in shares)						(22,837)
Forfeited, weighted average exercise price (in dollars per share)						$ 9.74
Ending balance, outstanding, number of options (in shares)	516,109					516,109		600,571
Ending balance, outstanding, weighted average exercise price (in dollars per share)	$ 29.51					$ 29.51		$ 25.31
Exercisable, number of options (in shares)	280,668					280,668
Weighted average exercise price (in dollars per share)	$ 21.83					$ 21.83
Exercisable, weighted average remaining contractual term (Year)						4 years 6 months 29 days
Exercisable, aggregate intrinsic value	$ 6,081					$ 6,081