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Marketable Securities - Available-For-Sale Securities (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]      
Debt Securities, Available-for-sale, Allowance for Credit Loss $ 0    
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale   $ 0  
Interest Income, Other $ 800,000 $ 3,400,000 $ 2,300,000