Marketable Securities - Summary of Cash Equivalents and Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	$ 80,968	$ 140,418
Available-for-sale securities, Amortized Cost	270,034	259,974
Available-for-sale securities, Gross Unrealized Gains	19
Available-for-sale securities, Gross Unrealized (Losses)	(25)	(259)
Available-for-sale securities, Estimated Fair Value	270,028	259,715
Total cash equivalents and available-for-sale securities, Amortized Cost	290,257	363,265
Total cash equivalents and available-for-sale securities, Gross Unrealized Gains	19
Total cash equivalents and available-for-sale securities, Gross Unrealized (Losses)	(25)	(259)
Total cash equivalents and available-for-sale securities, Estimated Fair Value	290,251	363,006
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	15,233	103,291
Cash equivalents, Estimated Fair Value	15,233	103,291
Commercial paper securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	4,990
Cash equivalents, Estimated Fair Value	4,990
Available-for-sale securities, Amortized Cost	180,380	177,353
Available-for-sale securities, Gross Unrealized Gains	19
Available-for-sale securities, Gross Unrealized (Losses)		(129)
Available-for-sale securities, Estimated Fair Value	180,399	177,224
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	20,223	103,291
Cash equivalents, Estimated Fair Value	20,223	103,291
Corporate debt securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	70,973	63,981
Available-for-sale securities, Gross Unrealized (Losses)	(25)	(111)
Available-for-sale securities, Estimated Fair Value	70,948	63,870
U.S. government sponsored entity debt securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	18,681	18,640
Available-for-sale securities, Gross Unrealized (Losses)		(19)
Available-for-sale securities, Estimated Fair Value	$ 18,681	$ 18,621

Cash equivalents and available for sale securities.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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