Fair Value of Financial Instruments - Summary of Composition of Short and Long Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 745,180	$ 492,537
Unrealized Gains	458	59
Unrealized Losses	(28)	(385)
Aggregate Fair Value	745,610	492,211
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,214	8,256
Unrealized Losses		(14)
Aggregate Fair Value	77,214	8,242
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,900	70,958
Unrealized Gains	251	3
Unrealized Losses	(13)	(233)
Aggregate Fair Value	87,138	70,728
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	581,066	413,323
Unrealized Gains	207	56
Unrealized Losses	(15)	(138)
Aggregate Fair Value	$ 581,258	$ 413,241

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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