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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.

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481830/320-10-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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