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Cash and Marketable Securities Summary of cost and estimated fair value of short-term investments (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Cash, Cash Equivalents, and Short-term Investments [Abstract]    
Maturing in one year or less $ 271,877 $ 259,715
Maturing after one year through five years 8,450 0
Total $ 280,327 $ 259,715