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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
Jun. 30, 2019
Dec. 31, 2018
Fair Value Measurement of Assets and Liabilities [Abstract]    
One year or less $ 1,552,909  
One year to two years 203,672  
Two years to three years 52  
Three years to four years 54  
Four years to five years 79  
Total $ 1,756,766 $ 917,796