Note 6 - Stock-based Compensation - Summary of Options Outstanding (Details) |
12 Months Ended |
|---|---|
|
Mar. 31, 2018
$ / shares
shares
| |
| Number outstanding (in shares) | shares | 22,000 |
| Weighted average exercise price (in dollars per share) | $ 61.19 |
| Weighted remaining contractual life (Year) | 6 years 219 days |
| Range One [Member] | |
| Ranges of exercise prices, lower limit (in dollars per share) | $ 42.45 |
| Ranges of exercise prices, upper limit (in dollars per share) | $ 54.11 |
| Number outstanding (in shares) | shares | 4,000 |
| Weighted average exercise price (in dollars per share) | $ 49.37 |
| Weighted remaining contractual life (Year) | 3 years 146 days |
| Range Two [Member] | |
| Ranges of exercise prices, lower limit (in dollars per share) | $ 56.76 |
| Ranges of exercise prices, upper limit (in dollars per share) | $ 76.13 |
| Number outstanding (in shares) | shares | 18,000 |
| Weighted average exercise price (in dollars per share) | $ 63.81 |
| Weighted remaining contractual life (Year) | 7 years 146 days |