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Note 10 - Stock-based Compensation - Summary of Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2014
Outstanding, Numbers of Shares (in shares) 515,720 437,248 398,172          
Outstanding, Weighted average exercise price (in dollars per share) $ 64.32 $ 55.81 $ 38.75          
Outstanding, Weighted average exercise price (in dollars per share) $ 64.32 $ 55.81 $ 38.75 $ 38.75 $ 75.78 $ 64.32 $ 55.81 $ 38.75
Outstanding, Weighted average remaining contractual term (Year) 5 years 5 years 73 days 4 years 328 days 4 years 146 days        
Outstanding, Aggregate intrinsic value         $ 23,956 $ 16,561 $ 9,445 $ 20,505
Granted, Numbers of Shares (in shares) 134,955 184,030 147,720          
Granted, Weighted average exercise price (in dollars per share) $ 102.52 $ 72.89 $ 88.62          
Granted, Weighted average remaining contractual term (Year) 5 years 182 days 6 years 255 days 7 years          
Forfeited, Numbers of Shares (in shares) (35,015) (16,334) (26,466)          
Forfeited, Weighted average exercise price (in dollars per share) $ 87.50 $ 75.16 $ 64.62          
Outstanding, Aggregate intrinsic value         23,956 16,561 9,445 $ 20,505
Forfeited, Weighted average remaining contractual term (Year) 4 years 182 days 6 years 182 days            
Expired, Numbers of Shares (in shares) (904)          
Expired, Weighted average exercise price (in dollars per share) $ 41.51          
Exercised, Numbers of Shares (in shares) (104,395) (89,224) (82,178)          
Exercised, Weighted average exercise price (in dollars per share) $ 50.12 $ 38.28 $ 28.87          
Outstanding, Numbers of Shares (in shares) 510,361 515,720 437,248 398,172        
Outstanding, Weighted average exercise price (in dollars per share) $ 75.78 $ 64.32 $ 55.81 $ 38.75        
Exercisable, Weighted average remaining contractual term (Year) 3 years 328 days 3 years 219 days 3 years 219 days          
Exercisable, Aggregate intrinsic value         $ 9,847 $ 8,481 $ 6,341  
Exercisable, Numbers of Shares (in shares)         136,595 157,457 163,210  
Exercisable, Weighted average exercise price (in dollars per share)         $ 50.61 $ 42.49 $ 33.35