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Marketable Securities and Fair Value Measurements - Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Investments, All Other Investments [Abstract]    
One year or less $ 98,460 $ 272,031
One year through two years 9,112 39,948
Marketable Securities $ 107,572 $ 311,979