Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 880,393	$ 732,348
Gross Unrealized Gains	1,449	30
Gross Unrealized Losses	(154)	(545)
Fair Value	881,688	731,833
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	205,172	220,531
Gross Unrealized Gains	176	8
Gross Unrealized Losses	(20)	(57)
Fair Value	205,328	220,482
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	429,148	258,876
Gross Unrealized Gains	797	6
Gross Unrealized Losses	(100)	(316)
Fair Value	429,845	258,566
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	142,568	78,600
Gross Unrealized Gains	457
Gross Unrealized Losses	(27)	(132)
Fair Value	142,998	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,252	77,630
Gross Unrealized Gains	14	8
Gross Unrealized Losses	(5)	(27)
Fair Value	52,261	77,611
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,253	96,711
Gross Unrealized Gains	5	8
Gross Unrealized Losses	(2)	(13)
Fair Value	$ 51,256	$ 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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