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Fair Value of Financial Instruments - Marketable securities portfolio (Details) - Debt securities (government) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Summary of marketable security portfolio    
Amortized Cost $ 292,287 $ 288,199
Net Unrealized Gains 138 170
Estimated Fair Value $ 292,425 $ 288,369
Minimum    
Summary of marketable security portfolio    
Contractual maturity dates 12 months  
Maximum    
Summary of marketable security portfolio    
Contractual maturity dates 18 months