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Note 3 - Fair Value of Financial Instruments - Financial Assets Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2022
Dec. 31, 2021
Money Market Funds [Member]    
Money market funds, Fair Value $ 76,094 $ 70,978
Fair Value, Recurring [Member]    
Assets, Fair Value 109,697 109,930
Fair Value, Recurring [Member] | US Treasury Securities [Member]    
Investments, Fair Value Disclosure 20,319 24,997
Fair Value, Recurring [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]    
Investments, Fair Value Disclosure 6,882 7,491
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]    
Investments, Fair Value Disclosure 6,402 6,464
Fair Value, Recurring [Member] | Money Market Funds [Member]    
Money market funds, Fair Value 76,094 70,978
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Assets, Fair Value 76,094 70,978
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]    
Investments, Fair Value Disclosure 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]    
Investments, Fair Value Disclosure 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]    
Investments, Fair Value Disclosure 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]    
Money market funds, Fair Value 76,094 70,978
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Assets, Fair Value 33,603 38,952
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]    
Investments, Fair Value Disclosure 20,319 24,997
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]    
Investments, Fair Value Disclosure 6,882 7,491
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]    
Investments, Fair Value Disclosure 6,402 6,464
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]    
Money market funds, Fair Value 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Assets, Fair Value 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]    
Investments, Fair Value Disclosure 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]    
Investments, Fair Value Disclosure 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]    
Investments, Fair Value Disclosure 0 0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]    
Money market funds, Fair Value $ 0 $ 0