Note 3 - Fair Value of Financial Instruments - Financial Assets Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Money Market Funds [Member]
Money market funds, Fair Value	$ 27,975	$ 30,834
Fair Value, Recurring [Member]
Investments, Fair Value Disclosure	47,774	80,538
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Investments, Fair Value Disclosure	19,799	41,542
Fair Value, Recurring [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments, Fair Value Disclosure		5,674
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Investments, Fair Value Disclosure		2,488
Fair Value, Recurring [Member] | Money Market Funds [Member]
Money market funds, Fair Value	27,975	30,834
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Investments, Fair Value Disclosure	27,975	30,834
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
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Investments, Fair Value Disclosure		0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Money market funds, Fair Value	27,975	30,834
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Investments, Fair Value Disclosure	19,799	49,704
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Investments, Fair Value Disclosure	19,799	41,542
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments, Fair Value Disclosure		5,674
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Investments, Fair Value Disclosure		2,488
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]
Investments, Fair Value Disclosure	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
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Investments, Fair Value Disclosure		0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Money market funds, Fair Value	$ 0	$ 0