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Note 3 - Fair Value Measurements (Details Textual) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Mar. 31, 2020
Sep. 30, 2020
Aug. 12, 2019
Money Market Funds, at Carrying Value $ 222,819   $ 66,735    
Proceeds from Convertible Debt $ 0 $ 167,070      
The Notes [Member]          
Debt Instrument, Interest Rate, Stated Percentage   1.375%      
Senior Notes [Member] | The Notes [Member]          
Proceeds from Convertible Debt     $ 172,500    
Debt Instrument, Interest Rate, Stated Percentage     1.375% 1.375% 1.375%