Note 5 - Stock-based Compensation - Summary of Stock Options (Details) - $ / shares	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Option Shares Reserved [Member]
Option shares reserved (in shares)	124,730	131,230	135,230
Granted (in shares)	(6,500)	(4,000)
Granted (in shares)	6,500	4,000
Exercised (in shares)
Exercised (in shares)
Share-based Payment Arrangement, Option [Member]
Option shares reserved (in shares)	30,500	24,000	22,000
Weighted average option exercise price (in dollars per share)	$ 68.28	$ 71.58	$ 70.89
Granted (in shares)	(6,500)	(4,000)
Granted (in shares)	6,500	4,000
Granted, weighted average option exercise price (in dollars per share)	$ 56.12	$ 67.65
Exercised (in shares)		2,000
Exercised (in shares)		(2,000)
Exercised, weighted average option exercise price (in dollars per share)		$ 56.18