Note 9 - Share-based Compensation - Summary of the Weighted-average Characteristics of Outstanding Stock Options (Details)	3 Months Ended
	Jun. 30, 2018 $ / shares shares
Outstanding Options Number of Shares (in shares) | shares	581,400
Outstanding Options Remaining Life (Year)	4 years 255 days
Outstanding Options Weighted Average Price (in dollars per share)	$ 4.07
Exercisable Options Number of Shares (in shares) | shares	461,400
Exercisable Options Weighted Average Price (in dollars per share)	$ 4.21
Range 01 [Member]
Lower Range of Exercise (in dollars per share)	1.60
Upper Range of Exercise (in dollars per share)	$ 3.70
Outstanding Options Number of Shares (in shares) | shares	166,800
Outstanding Options Remaining Life (Year)	5 years 36 days
Outstanding Options Weighted Average Price (in dollars per share)	$ 3.06
Exercisable Options Number of Shares (in shares) | shares	91,800
Exercisable Options Weighted Average Price (in dollars per share)	$ 2.82
Range 02 [Member]
Lower Range of Exercise (in dollars per share)	3.71
Upper Range of Exercise (in dollars per share)	$ 4.42
Outstanding Options Number of Shares (in shares) | shares	239,100
Outstanding Options Remaining Life (Year)	4 years 182 days
Outstanding Options Weighted Average Price (in dollars per share)	$ 3.83
Exercisable Options Number of Shares (in shares) | shares	194,100
Exercisable Options Weighted Average Price (in dollars per share)	$ 3.83
Range 03 [Member]
Lower Range of Exercise (in dollars per share)	4.43
Upper Range of Exercise (in dollars per share)	$ 5.40
Outstanding Options Number of Shares (in shares) | shares	95,000
Outstanding Options Remaining Life (Year)	4 years 328 days
Outstanding Options Weighted Average Price (in dollars per share)	$ 5
Exercisable Options Number of Shares (in shares) | shares	95,000
Exercisable Options Weighted Average Price (in dollars per share)	$ 5
Range 04 [Member]
Lower Range of Exercise (in dollars per share)	5.41
Upper Range of Exercise (in dollars per share)	$ 7.08
Outstanding Options Number of Shares (in shares) | shares	80,500
Outstanding Options Remaining Life (Year)	4 years 146 days
Outstanding Options Weighted Average Price (in dollars per share)	$ 5.77
Exercisable Options Number of Shares (in shares) | shares	80,500
Exercisable Options Weighted Average Price (in dollars per share)	$ 5.77