Note 10 - Stock-based Compensation - Stock Option Valuation Assumptions (Details) |
12 Months Ended | ||
|---|---|---|---|
Mar. 31, 2017 |
Mar. 31, 2016 |
Mar. 31, 2015 |
|
| Risk-free interest rate | 1.22% | ||
| Expected option life (years) (Year) | 5 years | ||
| Dividend yield | 0.55% | 0.70% | 0.90% |
| Minimum [Member] | |||
| Volatility | 32.05% | 27.10% | 24.40% |
| Risk-free interest rate | 1.90% | ||
| Expected option life (years) (Year) | 6 years | ||
| Maximum [Member] | |||
| Volatility | 33.76% | 30.20% | 27.10% |
| Risk-free interest rate | 2.30% | ||
| Expected option life (years) (Year) | 8 years | ||