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Note 12 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options outstanding (in shares) 569,639 766,042 998,987
Options outstanding, weighted- average exercise price (in dollars per share) $ 45 $ 36.52 $ 26.49
Granted (in shares) 60,100 117,480 110,335
Granted, weighted- average exercise price (in dollars per share) $ 69.06 $ 57.99 $ 67.36
Exercised (in shares) (187,313) (226,164) (316,761)
Exercised, weighted- average exercise price (in dollars per share) $ 35.93 $ 16.65 $ 14.36
Cancelled (in shares) (43,371) (87,719) (26,519)
Cancelled, weighted- average exercise price (in dollars per share) $ 61.02 $ 61.41 $ 51.72
Options outstanding (in shares) 399,055 569,639 766,042
Options outstanding, weighted- average exercise price (in dollars per share) $ 51.13 $ 45 $ 36.52
Options exercisable (in shares) 239,482    
Options exercisable, weighted- average exercise price (in dollars per share) $ 42.08