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Note 9 - Stock-based Compensation - Stock Option Activity (Details)
 6 Months Ended
Jun. 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 34,386
Granted, weighted- average exercise price (in dollars per share) | $ / shares $ 109.91
Exercised (in shares) | shares (108,941)
Exercised, weighted- average exercise price (in dollars per share) | $ / shares $ 37.04
Forfeited (in shares) | shares (17,077)
Forfeited, weighted- average exercise price (in dollars per share) | $ / shares $ 61.30
Options outstanding (in shares) | shares 307,765
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares $ 62.14
Options exercisable (in shares) | shares 168,243
Options exercisable, weighted- average exercise price (in dollars per share) | $ / shares $ 49.52