Note 5 - Stock-based Compensation - Summary of Options Outstanding (Details)	12 Months Ended
	Mar. 31, 2022 $ / shares shares
Number outstanding (in shares) | shares	29,000
Weighted average exercise price (in dollars per share)	$ 69.52
Weighted average contractual life (Year)	5 years
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)	$ 57.46
Number outstanding (in shares) | shares	17,500
Weighted average exercise price (in dollars per share)	$ 55.94
Weighted average contractual life (Year)	4 years
Range Two [Member]
Range of exercise prices, lower limit (in dollars per share)	$ 67.65
Range of exercise prices, upper limit (in dollars per share)	$ 107.86
Number outstanding (in shares) | shares	11,500
Weighted average exercise price (in dollars per share)	$ 78.44
Weighted average contractual life (Year)	5 years 8 months 12 days