Note 5 - Stock-based Compensation - Summary of Stock Options (Details) - $ / shares	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Option Shares Reserved [Member]
Option shares reserved (in shares)	135,230	139,230	143,230
Granted (in shares)	(4,000)	(4,000)
Granted (in shares)	4,000	4,000
Exercised (in shares)
Exercised (in shares)
Employee Stock Option [Member]
Option shares reserved (in shares)	22,000	22,000	19,000
Weighted average option exercise price (in dollars per share)	$ 70.89	$ 61.19	$ 57.51
Granted (in shares)	(4,000)	(4,000)
Granted (in shares)	4,000	4,000
Granted, weighted average option exercise price (in dollars per share)	$ 107.86	$ 76.13
Exercised (in shares)	4,000	1,000
Exercised (in shares)	(4,000)	(1,000)
Exercised, weighted average option exercise price (in dollars per share)	$ 54.49	$ 51.04