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Note 9 - Stock Option Valuation Assumptions (Details)
12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2014
Minimum [Member]      
Volatility 27.10% 24.40% 26.00%
Risk-free interest rate   1.90% 0.80%
Expected option life (years)   6 years 5 years
Maximum [Member]      
Volatility 30.20% 27.10% 28.70%
Risk-free interest rate   2.30% 2.10%
Expected option life (years)   8 years 10 years
Risk-free interest rate 1.09%    
Expected option life (years) 8 years    
Dividend yield 0.70% 0.90% 1.10%