Note 4 - Marketable Securities - Fair Value and Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair value	$ 27,735	$ 0
Gross unrealized losses	(20)
Maturity, Less Than 12 Months [Member]
Fair value	32,982
Gross unrealized losses	(20)
Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	0
Corporate Debt Securities [Member]
Gross unrealized losses	(12)
Corporate Debt Securities [Member] | Maturity, Less Than 12 Months [Member]
Fair value	17,653
Gross unrealized losses	(12)
Corporate Debt Securities [Member] | Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	0
Commercial Paper [Member]
Gross unrealized losses	0
Commercial Paper [Member] | Maturity, Less Than 12 Months [Member]
Fair value	7,768
Gross unrealized losses	0
Commercial Paper [Member] | Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	0
Money Market Funds [Member]
Gross unrealized losses	0
Money Market Funds [Member] | Maturity, Less Than 12 Months [Member]
Fair value	3,751
Gross unrealized losses	0
Money Market Funds [Member] | Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	0
Debt Security, Corporate, Non-US [Member]
Gross unrealized losses	(8)
Debt Security, Corporate, Non-US [Member] | Maturity, Less Than 12 Months [Member]
Fair value	2,873
Gross unrealized losses	(8)
Debt Security, Corporate, Non-US [Member] | Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	0
Municipal Bonds [Member]
Gross unrealized losses	0
Municipal Bonds [Member] | Maturity, Less Than 12 Months [Member]
Fair value	937
Gross unrealized losses	0
Municipal Bonds [Member] | Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	$ 0