Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 732,348	$ 212,642
Gross Unrealized Gains	30	2
Gross Unrealized Losses	(545)	(31)
Fair Value	731,833	212,613
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	220,531	49,612
Gross Unrealized Gains	8
Gross Unrealized Losses	(57)	(6)
Fair Value	220,482	49,606
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	258,876	48,982
Gross Unrealized Gains	6	2
Gross Unrealized Losses	(316)	(25)
Fair Value	258,566	48,959
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78,600
Gross Unrealized Losses	(132)
Fair Value	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,630	65,583
Gross Unrealized Gains	8
Gross Unrealized Losses	(27)
Fair Value	77,611	65,583
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	96,711	48,465
Gross Unrealized Gains	8
Gross Unrealized Losses	(13)
Fair Value	$ 96,706	$ 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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