Note 4 - Fair Value Measurements - Investments (Details) - USD ($) $ in Thousands		Mar. 31, 2021	Mar. 31, 2020
Adjusted cost	[1]	$ 66,239	$ 68,401
Gross unrealized losses	[1]	(18)	(11)
Fair value	[1]	66,221	68,390
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Adjusted cost	[1]	22,553	21,042
Gross unrealized losses	[1]	0	0
Fair value	[1]	22,553	21,042
Bank Time Deposits [Member] | Fair Value, Inputs, Level 2 [Member]
Adjusted cost	[1]	5,000	5,458
Gross unrealized losses	[1]	0	0
Fair value	[1]	5,000	5,458
Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Adjusted cost	[1]	18,455	30,280
Gross unrealized losses	[1]	(7)	(5)
Fair value	[1]	18,448	30,275
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Adjusted cost	[1]	20,231	11,621
Gross unrealized losses	[1]	(11)	(6)
Fair value	[1]	$ 20,220	$ 11,615

[1] FASB Topic 820, Fair Value Measurements, establishes a fair value hierarchy that requires the use of observable market data, when available, and prioritizes the inputs to valuation techniques used to measure fair value in the following categories: Level 1: Quoted market prices in active markets for identical assets or liabilities. Level 2: Observable market-based inputs or unobservable inputs that are corroborated by market data. Level 3: Unobservable inputs that are not corroborated by market data.