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Note 9 - Stock-based Compensation - Non-vested Restricted Stock Options (Details) - Restricted Stock [Member]
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Restricted stock (in shares) | shares 334,608
Restricted stock, weighted-average grant date fair value (in dollars per share) | $ / shares $ 63.29
Granted (in shares) | shares 20,976
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares $ 105.75
Restrictions lapsed (in shares) | shares (22,616)
Restrictions lapsed, weighted-average grant date fair value (in dollars per share) | $ / shares $ 62.94
Forfeited (in shares) | shares (13,536)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares $ 61.33
Restricted stock (in shares) | shares 319,432
Restricted stock, weighted-average grant date fair value (in dollars per share) | $ / shares $ 66.18