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Note 7 - Stock-based Compensation - Summary of Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Dec. 31, 2017
Mar. 31, 2017
Outstanding (in shares) 510,361  
Outstanding (in dollars per share) $ 75.78  
Outstanding (Year) 4 years 182 days 5 years
Outstanding $ 20,092 $ 23,956
Stock options granted (in shares) 95,605  
Stock options granted (in dollars per share) $ 123.09  
Stock options granted (Year) 5 years 109 days  
Stock options forfeited (in shares) (52,833)  
Stock options forfeited (in dollars per share) $ 95.29  
Stock options forfeited (Year) 4 years 292 days  
Stock options expired (in shares) (964)  
Stock options expired (in dollars per share) $ 79.87  
Stock options expired (Year) 3 years 36 days  
Stock options exercised (in shares) (51,496)  
Stock options exercised (in dollars per share) $ 61.27  
Outstanding (in shares) 500,673 510,361
Outstanding (in dollars per share) $ 84.24 $ 75.78
Exercisable (in shares) 164,475  
Exercisable (in dollars per share) $ 60.13  
Exercisable (Year) 3 years 292 days  
Exercisable $ 10,554