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Financial Instruments - Estimated Fair Value of Marketable Debt Securities by Contractual Maturity (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]    
Due in one year or less $ 77,263  
Due in one to two years 40,983  
Total $ 118,246 $ 45,802