Quarterly report pursuant to Section 13 or 15(d)

Note 4 - Marketable Securities - Fair Value of Marketable Securities (Details)

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Note 4 - Marketable Securities - Fair Value of Marketable Securities (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Total marketable securities $ 31,878 $ 20,098
Fair Value, Inputs, Level 1 [Member]    
Total marketable securities 0 0
Fair Value, Inputs, Level 2 [Member]    
Total marketable securities 31,878 20,098
Fair Value, Inputs, Level 3 [Member]    
Total marketable securities 0 0
US Government Agencies Debt Securities [Member]    
Total marketable securities 8,270 8,924
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Total marketable securities 0 0
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Total marketable securities 8,270 8,924
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Total marketable securities 0 0
Bank Certificates of Deposit [Member]    
Total marketable securities 0 496
Bank Certificates of Deposit [Member] | Fair Value, Inputs, Level 1 [Member]    
Total marketable securities 0 0
Bank Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]    
Total marketable securities 0 496
Bank Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]    
Total marketable securities 0 0
Commercial Paper and Corporate Bonds [Member]    
Total marketable securities 23,608 10,678
Commercial Paper and Corporate Bonds [Member] | Fair Value, Inputs, Level 1 [Member]    
Total marketable securities 0 0
Commercial Paper and Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]    
Total marketable securities 23,608 10,678
Commercial Paper and Corporate Bonds [Member] | Fair Value, Inputs, Level 3 [Member]    
Total marketable securities $ 0 $ 0