Note 9 - Employee Equity Incentive Plans - Fair Value and Assumptions (Details) - $ / shares		3 Months Ended		6 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Stock Option [Member]
Weighted average estimated fair value of grant per share (in dollars per share)	[1]		$ 4.99	$ 6.49	$ 4.40
Risk-free interest rate	[1]		1.30%	1.90%	1.30%
Expected term in years (Year)	[1]		6 years 149 days	6 years 229 days	6 years 149 days
Volatility	[1]		42.00%	36.80%	41.40%
Dividend yield	[1],[2]		0.00%	0.00%	0.80%
Employee Stock Purchase Plan 1999 [Member]
Weighted average estimated fair value of grant per share (in dollars per share)	[3]			$ 2.92	$ 3.94
Risk-free interest rate	[3]			0.70%	0.30%
Expected term in years (Year)	[3]			273 days	1 year
Volatility	[3]			29.90%	42.90%
Dividend yield	[2],[3]			0.00%	1.40%

[1] No stock options were granted during the quarter ended September 30, 2017.

[2] No dividends were declared for the six months ended September 30, 2017 and the three months ended September 30, 2016.

[3] Under the stock purchase plan, rights to purchase shares are only granted during the first and third quarters of each fiscal year.