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Fair Value Measurement of Assets and Liabilities (Estimated Fair Values Of Cash Equivalents, Short-Term and Long-Term Investments, Effective Maturities) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value Measurement of Assets and Liabilities [Abstract]    
One year or less $ 321,470  
One year to two years 213  
Two years to three years 191  
Three years to four years 24  
Four years to five years 5  
Total $ 321,903 $ 125,546