Note 5 - Stock-based Compensation - Summary of Stock Options (Details) - $ / shares	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Option Shares Reserved [Member]
Option shares reserved (in shares)	131,230	135,230	139,230
Granted (in shares)	(4,000)	(4,000)
Granted (in shares)	4,000	4,000
Exercised (in shares)
Exercised (in shares)
Share-based Payment Arrangement, Option [Member]
Option shares reserved (in shares)	24,000	22,000	22,000
Weighted average option exercise price (in dollars per share)	$ 71.58	$ 70.89	$ 61.19
Granted (in shares)	(4,000)	(4,000)
Granted (in shares)	4,000	4,000
Granted, weighted average option exercise price (in dollars per share)	$ 67.65	$ 107.86
Exercised (in shares)	2,000	4,000
Exercised (in shares)	(2,000)	(4,000)
Exercised, weighted average option exercise price (in dollars per share)	$ 56.18	$ 54.49