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Note 6 - Stock-based Compensation - Summary of Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Sep. 30, 2017
Mar. 31, 2017
Outstanding (in shares) 510,361  
Outstanding (in dollars per share) $ 75.78  
Outstanding (Year) 4 years 292 days 5 years
Outstanding $ 33,320 $ 23,956
Stock options granted (in shares) 95,605  
Stock options granted (in dollars per share) $ 123.09  
Stock options granted (Year) 5 years 182 days  
Stock options forfeited (in shares) (40,661)  
Stock options forfeited (in dollars per share) $ 93.74  
Stock options forfeited (Year) 5 years 36 days  
Stock options expired (in shares) (96)  
Stock options expired (in dollars per share) $ 97.78  
Stock options expired (Year) 4 years 182 days  
Stock options exercised (in shares) (50,503)  
Stock options exercised (in dollars per share) $ 61.09  
Outstanding (in shares) 514,706 510,361
Outstanding (in dollars per share) $ 84.58 $ 75.78
Exercisable (in shares) 162,831  
Exercisable (in dollars per share) $ 60.05  
Exercisable (Year) 4 years  
Exercisable $ 14,536