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Marketable Securities - Available-For-Sale Securities (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]      
Available-for-sale securities, other than temporary impairment losses $ 0 $ 0  
Interest Income, Other $ 2,300,000 $ 2,400,000 $ 2,300,000