Note 5 - Stock-based Compensation - Summary of Options Outstanding (Details)	12 Months Ended
	Mar. 31, 2021 $ / shares shares
Number outstanding (in shares) | shares	30,500
Weighted average exercise price (in dollars per share)	$ 68.28
Weighted average contractual life (Year)	5 years 255 days
Range One [Member]
Range of exercise prices, lower limit (in dollars per share)	$ 49.86
Range of exercise prices, upper limit (in dollars per share)	$ 67.69
Number outstanding (in shares) | shares	22,000
Weighted average exercise price (in dollars per share)	$ 59.85
Weighted average contractual life (Year)	5 years 109 days
Range Two [Member]
Range of exercise prices, lower limit (in dollars per share)	$ 76.13
Range of exercise prices, upper limit (in dollars per share)	$ 107.86
Number outstanding (in shares) | shares	8,000
Weighted average exercise price (in dollars per share)	$ 92
Weighted average contractual life (Year)	6 years 328 days