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Note 2 - Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2022
Dec. 31, 2021
Money market funds $ 67,811 $ 75,258
Commercial paper 23,541 18,896
Corporate bonds 86,107 86,703
Asset-backed securities 22,099 18,863
U.S. Treasury and government agencies 249,412 254,527
Foreign government 1,002 1,019
Total 449,972 455,266
Fair Value, Inputs, Level 1 [Member]    
Money market funds 67,811 75,258
Commercial paper 0 0
Corporate bonds 0 0
Asset-backed securities 0 0
U.S. Treasury and government agencies 0 0
Foreign government 0 0
Total 67,811 75,258
Fair Value, Inputs, Level 2 [Member]    
Money market funds 0 0
Commercial paper 23,541 18,896
Corporate bonds 86,107 86,703
Asset-backed securities 22,099 18,863
U.S. Treasury and government agencies 249,412 254,527
Foreign government 1,002 1,019
Total $ 382,161 $ 380,008