Note 4 - Marketable Securities - Fair Value and Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair value	$ 45,960	$ 0
Gross unrealized losses	(11)
Maturity, Less Than 12 Months [Member]
Fair value	63,012
Gross unrealized losses	(11)
Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	0
Corporate Debt Securities [Member]
Gross unrealized losses	(10)
Corporate Debt Securities [Member] | Maturity, Less Than 12 Months [Member]
Fair value	30,707
Gross unrealized losses	(10)
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	11,465
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	17,053
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	(1)
Debt Security, Corporate, Non-US [Member] | Maturity, Less Than 12 Months [Member]
Fair value	2,849
Gross unrealized losses	(1)
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	938
Gross unrealized losses	0
Municipal Bonds [Member] | Maturity, Greater Than 12 Months [Member]
Fair value	0
Gross unrealized losses	$ 0