Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,009,062	$ 732,348
Gross Unrealized Gains	252	30
Gross Unrealized Losses	(358)	(545)
Fair Value	1,008,956	731,833
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	248,082	220,531
Gross Unrealized Gains	86	8
Gross Unrealized Losses	(13)	(57)
Fair Value	248,155	220,482
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	386,333	258,876
Gross Unrealized Gains	119	6
Gross Unrealized Losses	(189)	(316)
Fair Value	386,263	258,566
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	155,571	78,600
Gross Unrealized Gains	42
Gross Unrealized Losses	(96)	(132)
Fair Value	155,517	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	108,487	77,630
Gross Unrealized Gains	5	8
Gross Unrealized Losses	(53)	(27)
Fair Value	108,439	77,611
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110,589	96,711
Gross Unrealized Gains		8
Gross Unrealized Losses	(7)	(13)
Fair Value	$ 110,582	$ 96,706