Note 6 - Stock-based Compensation - Summary of Stock Options (Details) - $ / shares	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Option Shares Reserved [Member]
Option shares reserved (in shares)	139,230	143,230	147,230	151,230
Granted (in shares)	(4,000)	(4,000)	(4,000)
Granted (in shares)	4,000	4,000	4,000
Exercised (in shares)
Exercised (in shares)
Employee Stock Option [Member]
Option shares reserved (in shares)	22,000	19,000	21,000	35,090
Weighted average option exercise price (in dollars per share)	$ 61.19	$ 57.51	$ 52.85	$ 33.51
Granted (in shares)	(4,000)	(4,000)	(4,000)
Granted (in shares)	4,000	4,000	4,000
Granted, weighted average option exercise price (in dollars per share)	$ 76.13	$ 57.46	$ 56.76
Exercised (in shares)	1,000	6,000	18,090
Exercised (in shares)	(1,000)	(6,000)	(18,090)
Exercised, weighted average option exercise price (in dollars per share)	$ 51.04	$ 41.19	$ 16.19