Note 2 - Fair Value of Financial Instruments - Summary of the Fair Value of Marketable Securities (Details) $ in Thousands	Mar. 31, 2021 USD ($)
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	$ 252,181
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	68,014
Available-for-sale Securities, Debt and Equity Maturities, Year over two years, Fair Value	33,460
Debt Securities, Available-for-sale	353,655
Commercial Paper [Member]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	15,145
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	0
Available-for-sale Securities, Debt and Equity Maturities, Year over two years, Fair Value	0
Debt Securities, Available-for-sale	15,145
US Government Agencies Debt Securities [Member]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	211,969
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	11,019
Available-for-sale Securities, Debt and Equity Maturities, Year over two years, Fair Value	0
Debt Securities, Available-for-sale	222,988
Foreign Government Agencies [Member]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	0
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	1,028
Available-for-sale Securities, Debt and Equity Maturities, Year over two years, Fair Value	0
Debt Securities, Available-for-sale	1,028
Corporate Bond Securities [Member]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	21,968
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	43,855
Available-for-sale Securities, Debt and Equity Maturities, Year over two years, Fair Value	19,241
Debt Securities, Available-for-sale	85,064
Asset-backed Securities [Member]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	3,099
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	12,112
Available-for-sale Securities, Debt and Equity Maturities, Year over two years, Fair Value	14,219
Debt Securities, Available-for-sale	$ 29,430