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Fair Value Measurements - Schedule of Investments Classified as Available for Sale Securities with Contractual Maturities (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Cash and Cash equivalents $ 74,889 $ 98,426
Short-term investments (due within one year) 182,948 77,277 [1]
Long-term investments (due between one and two years) 7,746  
Total cash and investments in marketable securities $ 265,583 $ 175,703
[1] The Condensed Consolidated Balance Sheet as of December 31, 2017 has been derived from the audited financial statements as of that date.