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Note 10 - Stock Based Compensation - Summary of Stock Option Activity (Details) - Employee Stock Option [Member] - $ / shares
3 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Outstanding, beginning balance (in shares) 1,366,934 1,484,914
Granted (in shares) 14,265  
Exercised (in shares) (93,500) (112,669)
Forfeited or expired (in shares) (6,759) (5,311)
Outstanding, ending balance (in shares) 1,280,940 1,366,934
Exercisable (in shares) 544,596  
Outstanding, beginning balance, weighted average exercise price (in dollars per share) $ 33.20 $ 32.53
Granted, weighted average exercise price (in dollars per share) 53.70  
Exercised, weighted average exercise price (in dollars per share) 20.07 24.16
Forfeited or expired (in dollars per share) 35.06 36.94
Outstanding, Ending balance, weighted average exercise price (in dollars per share) 34.38 $ 33.20
Exercisable, weighted average exercise price (in dollars per share) $ 26.37