Marketable Securities - Schedule of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	$ 59,406	$ 49,826
Available-for-sale securities, Amortized Cost	514,348	194,749
Available-for-sale securities, Gross Unrealized Gains	110
Available-for-sale securities, Gross Unrealized (Losses)	(236)	(255)
Available-for-sale securities, Estimated Fair Value	514,222	194,494
Total cash equivalents and available-for-sale securities, Amortized Cost	571,489	223,634
Total cash equivalents and available-for-sale securities, Gross Unrealized Gains	111
Total cash equivalents and available-for-sale securities, Gross Unrealized (Losses)	(236)	(255)
Total cash equivalents and available-for-sale securities, Estimated Fair Value	571,364	223,379
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	41,909	24,290
Cash equivalents, Estimated Fair Value	41,909	24,290
Commercial paper securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	15,232	4,595
Cash equivalents, Gross Unrealized Gains	1
Cash equivalents, Estimated Fair Value	15,233	4,595
Available-for-sale securities, Amortized Cost	396,958	110,365
Available-for-sale securities, Gross Unrealized Gains	110
Available-for-sale securities, Gross Unrealized (Losses)	(75)	(118)
Available-for-sale securities, Estimated Fair Value	396,993	110,247
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	57,141	28,885
Cash equivalents, Gross Unrealized Gains	1
Cash equivalents, Estimated Fair Value	57,142	28,885
Corporate debt securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	98,834	75,886
Available-for-sale securities, Gross Unrealized (Losses)	(158)	(131)
Available-for-sale securities, Estimated Fair Value	98,676	75,755
U.S. government sponsored entity debt securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	18,556	8,498
Available-for-sale securities, Gross Unrealized (Losses)	(3)	(6)
Available-for-sale securities, Estimated Fair Value	$ 18,553	$ 8,492

Cash and cash equivalents accumulated gross unrealized gain before tax.

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