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Marketable Securities - Available-For-Sale Securities (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]      
Available-for-sale securities, other than temporary impairment losses $ 0 $ 0  
Amortized Cost      
Mature in one year or less   192,985,000  
Mature between one and five years   7,981,000  
Amortized Cost 93,455,000 200,966,000  
Fair Value      
Mature in one year or less   192,898,000  
Mature between one and five years   7,986,000  
Fair Value 93,394,000 200,884,000  
Interest Income, Other $ 2,400,000 $ 2,300,000 $ 200,000