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Note 4 - Fair-Value Measurements - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Short-term investments $ 164,743 $ 183,909
Restricted investments 1,054 1,054
Fair Value, Recurring [Member]    
Short-term investments 164,762 183,931
Restricted investments 1,054 1,054
Total 165,816 184,985
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Short-term investments 62,643 84,882
Restricted investments 1,054 1,054
Total 63,697 85,936
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Short-term investments 102,119 99,049
Restricted investments 0 0
Total 102,119 99,049
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Short-term investments 0 0
Restricted investments 0 0
Total 0 0
US Treasury Securities [Member] | Fair Value, Recurring [Member]    
Short-term investments 102,119 99,049
US Treasury Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Short-term investments 0 0
US Treasury Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Short-term investments 102,119 99,049
US Treasury Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Short-term investments 0 0
Money Market Funds [Member] | Fair Value, Recurring [Member]    
Short-term investments 62,643 84,882
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Short-term investments 62,643 84,882
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Short-term investments 0 0
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Short-term investments $ 0 $ 0