Fair Value of Financial Instruments - Summary of Composition of Short and Long Term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 804,636	$ 745,180
Unrealized Gains	1,656	458
Unrealized Losses	(680)	(28)
Aggregate Fair Value	805,612	745,610
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	83,100	77,214
Aggregate Fair Value	83,100	77,214
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	120,639	86,900
Unrealized Gains	234	251
Unrealized Losses	(672)	(13)
Aggregate Fair Value	120,201	87,138
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	600,897	581,066
Unrealized Gains	1,422	207
Unrealized Losses	(8)	(15)
Aggregate Fair Value	$ 602,311	$ 581,258

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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