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Note 10 - Stock-based Compensation - Stock Option Valuation Assumptions (Details)
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Risk-free interest rate 1.77% 1.22% 1.09%
Expected option life (years) (Year) 6 years 5 years 8 years
Dividend yield 0.57% 0.55% 0.70%
Minimum [Member]      
Volatility 32.85% 32.05% 27.10%
Maximum [Member]      
Volatility 35.44% 33.76% 30.20%