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Cash and Investments in Marketable Securities - Cash and Investments in Marketable Securities, Including Cash Equivalents (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Cash and cash equivalents $ 222,261 $ 4,762
Short-term investments 1,198,149 291,370
Long-term investments 619,140 57,088
Total cash and investments in marketable securities $ 2,039,550 $ 353,220