Note 9 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Options outstanding (in shares) | shares	229,531
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 86.46
Granted (in shares) | shares	57,901
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 128.14
Exercised (in shares) | shares	(28,991)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 58.78
Forfeited (in shares) | shares	(9,893)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 100.58
Options outstanding (in shares) | shares	248,548
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 98.84
Options exercisable (in shares) | shares	116,088
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 81.47