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Fair value of financial instruments - Marketable securities portfolio (Details) - Debt securities (government) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Summary of marketable security portfolio    
Amortized Cost $ 236,902 $ 284,795
Net Unrealized Gains 123 75
Estimated Fair Value $ 237,025 $ 284,870
Minimum    
Summary of marketable security portfolio    
Contractual maturity dates 12 months  
Maximum    
Summary of marketable security portfolio    
Contractual maturity dates 18 months