Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,068,789	$ 732,348
Gross Unrealized Gains	1,679	30
Gross Unrealized Losses	(104)	(545)
Fair Value	1,070,364	731,833
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	290,271	220,531
Gross Unrealized Gains	302	8
Gross Unrealized Losses	(2)	(57)
Fair Value	290,571	220,482
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	477,932	258,876
Gross Unrealized Gains	942	6
Gross Unrealized Losses	(68)	(316)
Fair Value	478,806	258,566
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	147,062	78,600
Gross Unrealized Gains	434
Gross Unrealized Losses	(26)	(132)
Fair Value	147,470	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	107,105	77,630
Gross Unrealized Gains	1	8
Gross Unrealized Losses	(7)	(27)
Fair Value	107,099	77,611
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,419	96,711
Gross Unrealized Gains		8
Gross Unrealized Losses	(1)	(13)
Fair Value	$ 46,418	$ 96,706

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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