Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 861,292	$ 732,348
Gross Unrealized Gains	1,866	30
Gross Unrealized Losses	(89)	(545)
Fair Value	863,069	731,833
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	207,183	220,531
Gross Unrealized Gains	217	8
Gross Unrealized Losses	(5)	(57)
Fair Value	207,395	220,482
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	394,825	258,876
Gross Unrealized Gains	1,045	6
Gross Unrealized Losses	(66)	(316)
Fair Value	395,804	258,566
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	157,241	78,600
Gross Unrealized Gains	557
Gross Unrealized Losses	(18)	(132)
Fair Value	157,780	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	53,505	77,630
Gross Unrealized Gains	36	8
Gross Unrealized Losses		(27)
Fair Value	53,541	77,611
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,538	96,711
Gross Unrealized Gains	11	8
Gross Unrealized Losses		(13)
Fair Value	$ 48,549	$ 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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