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Fair Value of Financial Instruments - Marketable securities portfolio (Details) - Debt securities (government) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Summary of marketable security portfolio    
Amortized Cost $ 283,380 $ 288,199
Net Unrealized Gains 5 170
Estimated Fair Value $ 283,385 $ 288,369
Minimum    
Summary of marketable security portfolio    
Contractual maturity dates 12 months  
Maximum    
Summary of marketable security portfolio    
Contractual maturity dates 18 months