Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 768,535	$ 212,642
Gross Unrealized Gains	70	2
Gross Unrealized Losses	(327)	(31)
Fair Value	768,278	212,613
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	283,701	49,612
Gross Unrealized Losses	(131)	(6)
Fair Value	283,570	49,606
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	152,848	48,982
Gross Unrealized Gains	5	2
Gross Unrealized Losses	(106)	(25)
Fair Value	152,747	48,959
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,610
Gross Unrealized Gains	1
Gross Unrealized Losses	(60)
Fair Value	89,551
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	103,516	65,583
Gross Unrealized Gains	34
Gross Unrealized Losses	(19)
Fair Value	103,531	65,583
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,860	48,465
Gross Unrealized Gains	30
Gross Unrealized Losses	(11)
Fair Value	$ 138,879	$ 48,465

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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