Note 6 - Stock-based Compensation - Summary of Options Outstanding (Details)	12 Months Ended
	Mar. 31, 2018 $ / shares shares
Number outstanding (in shares) | shares	22,000
Weighted average exercise price (in dollars per share)	$ 61.19
Weighted remaining contractual life (Year)	6 years 219 days
Range One [Member]
Ranges of exercise prices, lower limit (in dollars per share)	$ 42.45
Ranges of exercise prices, upper limit (in dollars per share)	$ 54.11
Number outstanding (in shares) | shares	4,000
Weighted average exercise price (in dollars per share)	$ 49.37
Weighted remaining contractual life (Year)	3 years 146 days
Range Two [Member]
Ranges of exercise prices, lower limit (in dollars per share)	$ 56.76
Ranges of exercise prices, upper limit (in dollars per share)	$ 76.13
Number outstanding (in shares) | shares	18,000
Weighted average exercise price (in dollars per share)	$ 63.81
Weighted remaining contractual life (Year)	7 years 146 days