Note 5 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Sep. 30, 2023	Dec. 31, 2022
Short-term investments		$ 94,180	$ 143,235
US Treasury Securities [Member]
Short-term investments		21,667	34,197
Corporate Debt Securities [Member]
Short-term investments	[1]	40,528	59,073
Asset-Backed Securities [Member]
Short-term investments		8,621	12,658
Certificates of Deposit [Member]
Short-term investments		22,639	36,479
Debt Security, Government, Non-US [Member]
Short-term investments		725	828
Fair Value, Recurring [Member]
Assets, Fair Value Disclosure		387,566	385,576
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Short-term investments		21,667	34,196
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Short-term investments		41,523	69,753
Fair Value, Recurring [Member] | Asset-Backed Securities [Member]
Short-term investments		8,621	12,658
Fair Value, Recurring [Member] | Certificates of Deposit [Member]
Short-term investments		22,639	37,480
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		725	828
Fair Value, Recurring [Member] | Cash [Member]
Cash and cash equivalents		243,956	190,371
Fair Value, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents		48,435	40,290
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure		243,956	190,371
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Asset-Backed Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents		243,956	190,371
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure		143,610	195,205
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments		21,667	34,196
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments		41,523	69,753
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Short-term investments		8,621	12,658
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments		22,639	37,480
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		725	828
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Cash and cash equivalents		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents		48,435	40,290
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Asset-Backed Securities [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Cash and cash equivalents		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents		$ 0	$ 0

[1] Corporate debt securities include investments in financial and other corporate institutions. No single issuer represents a significant portion of the total corporate debt securities portfolio.