MARKETABLE DEBT SECURITIES - Summary of Company's Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 968,656	$ 1,315,012
Gross Unrealized Gains	4,516	246
Gross Unrealized Losses	(334)	(109)
Fair Value	972,838	1,315,149
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114,592	146,016
Gross Unrealized Gains	9	58
Gross Unrealized Losses	(321)	(34)
Fair Value	114,280	146,040
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	105,028	32,487
Gross Unrealized Gains	275	3
Gross Unrealized Losses	(6)	(2)
Fair Value	105,297	32,488
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	722,103	1,092,293
Gross Unrealized Gains	4,232	185
Gross Unrealized Losses	0	(71)
Fair Value	726,335	1,092,407
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,141	4,303
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(2)
Fair Value	5,134	4,301
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,792	39,913
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 21,792	$ 39,913

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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