Note 4 - Fair Value Measurements - Investments (Details) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020
Adjusted cost	[1]	$ 66,537	$ 68,401
Gross unrealized losses	[1]	(2)	(11)
Fair value	[1]	66,535	68,390
Fair value	[1]	(66,535)	(68,390)
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Adjusted cost	[1]	42,735	21,042
Gross unrealized losses	[1]	0	0
Fair value	[1]	42,735	21,042
Fair value	[1]	(42,735)	(21,042)
Bank Time Deposits [Member] | Fair Value, Inputs, Level 2 [Member]
Adjusted cost	[1]	0	5,458
Gross unrealized losses	[1]	0	0
Fair value	[1]	0	5,458
Fair value	[1]	0	(5,458)
Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Adjusted cost	[1]	22,993	30,280
Gross unrealized losses	[1]	(2)	(5)
Fair value	[1]	22,991	30,275
Fair value	[1]	(22,991)	(30,275)
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Adjusted cost	[1]	809	11,621
Gross unrealized losses	[1]	0	(6)
Fair value	[1]	809	11,615
Fair value	[1]	$ (809)	$ (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.