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Note 9 - Stock-based Compensation - Stock Option Activity (Details)
 3 Months Ended
Mar. 31, 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 25,642
Granted, weighted-average exercise price (in dollars per share) | $ / shares $ 169.32
Exercised (in shares) | shares (28,991)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares $ 58.78
Forfeited (in shares) | shares (8,097)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares $ 101.21
Options outstanding (in shares) | shares 218,085
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares $ 99.34
Options exercisable (in shares) | shares 100,932
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares $ 77.31