Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 674,105	$ 880,393
Gross Unrealized Gains	1,306	1,449
Gross Unrealized Losses	(2,098)	(154)
Fair Value	673,313	881,688
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	145,202	205,172
Gross Unrealized Gains	711	176
Gross Unrealized Losses		(20)
Fair Value	145,913	205,328
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	363,156	429,148
Gross Unrealized Gains	415	797
Gross Unrealized Losses	(1,410)	(100)
Fair Value	362,161	429,845
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,878	142,568
Gross Unrealized Gains	180	457
Gross Unrealized Losses	(662)	(27)
Fair Value	116,396	142,998
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,930	52,252
Gross Unrealized Gains		14
Gross Unrealized Losses	(26)	(5)
Fair Value	27,904	52,261
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,939	51,253
Gross Unrealized Gains		5
Gross Unrealized Losses		(2)
Fair Value	$ 20,939	$ 51,256

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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