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Note 9 - Stock-based Compensation - Stock Option Activity (Details)
 9 Months Ended
Sep. 30, 2018
$ / shares
shares
Options outstanding (in shares) | shares 399,397
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares $ 51.14
Granted (in shares) | shares 36,258
Granted, weighted- average exercise price (in dollars per share) | $ / shares $ 110.59
Exercised (in shares) | shares (152,110)
Exercised, weighted- average exercise price (in dollars per share) | $ / shares $ 39.31
Forfeited (in shares) | shares (22,742)
Forfeited, weighted- average exercise price (in dollars per share) | $ / shares $ 70
Options outstanding (in shares) | shares 260,803
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares $ 64.66
Options exercisable (in shares) | shares 126,008
Options exercisable, weighted- average exercise price (in dollars per share) | $ / shares $ 51.25