Note 4 - Marketable Securities - Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost	$ 133,707	$ 151,965
Unrealized gains	7	64
Unrealized Losses	(437)	(366)
Fair Value	133,277	151,663
Marketable Securities	133,277	151,663
Corporate Debt Securities [Member]
Amortized Cost	41,850	87,998
Unrealized gains	5	12
Unrealized Losses	(222)	(250)
Fair Value	41,633	87,760
Marketable Securities	41,633	87,760
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Amortized Cost	48,912	37,680
Unrealized gains	0	33
Unrealized Losses	(23)	(31)
Fair Value	48,889	37,682
Marketable Securities	48,889	37,682
Certificates of Deposit [Member]
Amortized Cost	19,557	19,689
Unrealized gains	2	16
Unrealized Losses	(12)	(38)
Fair Value	19,547	19,667
Marketable Securities	19,547	19,667
Asset-Backed Securities [Member]
Amortized Cost	8,453	6,598
Unrealized gains	0	3
Unrealized Losses	(27)	(47)
Fair Value	8,426	6,554
Marketable Securities	8,426	$ 6,554
US Treasury Securities [Member]
Amortized Cost	14,935
Unrealized gains	0
Unrealized Losses	(153)
Fair Value	14,782
Marketable Securities	$ 14,782