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Note 2 - Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Money market funds $ 75,258 $ 38,028
Commercial paper 18,896 9,146
U.S. Treasury and government agencies 254,527 251,827
Foreign government 1,019 1,037
Corporate bonds 86,703 76,445
Asset-backed securities 18,863 44,894
Total 455,266 421,377
Fair Value, Inputs, Level 1 [Member]    
Money market funds 75,258 38,028
Commercial paper 0 0
U.S. Treasury and government agencies 0 0
Foreign government 0 0
Corporate bonds 0 0
Asset-backed securities 0 0
Total 75,258 38,028
Fair Value, Inputs, Level 2 [Member]    
Money market funds 0 0
Commercial paper 18,896 9,146
U.S. Treasury and government agencies 254,527 251,827
Foreign government 1,019 1,037
Corporate bonds 86,703 76,445
Asset-backed securities 18,863 44,894
Total $ 380,008 $ 383,349