Note 6 - Stock-based Compensation - Summary of Options Outstanding (Details)	12 Months Ended
	Mar. 31, 2017 $ / shares shares
Number Outstanding (in shares) | shares	19,000
Weighted Average Exercise Price (in dollars per share)	$ 57.51
Weighted Remaining Contractual Life (Year)	6 years 292 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	5,000
Weighted Average Exercise Price (in dollars per share)	$ 49.70
Weighted Remaining Contractual Life (Year)	4 years
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)	$ 67.69
Number Outstanding (in shares) | shares	14,000
Weighted Average Exercise Price (in dollars per share)	$ 60.30
Weighted Remaining Contractual Life (Year)	7 years 292 days