Note 6 - Financial Instruments and Investments in Securities - Investments in Held-to-Maturity Securities, Recorded at Amortized Cost (Details) - Short-term Investments [Member] - Time Deposits [Member] - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019
Cost	$ 390,170	$ 434,754
Gross Unrealized Gains	329	184
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 390,499	$ 434,938