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Note 5 - Marketable Securities (Details Textual) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale, Realized Loss $ (90) $ 0 $ 0
Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One 275,300    
Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year Two 64,400    
Interest and Dividend Income [Member]      
Investment Income, Interest 500 0 $ 0
Prepaid Expenses and Other Current Assets [Member]      
Interest Receivable, Current $ 1,500 $ 0