INVESTMENTS IN DEBT SECURITIES - Short-Term and Long-Term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,158,420
Fair Value	1,161,556
Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	760,203	$ 490,299
Gross Unrealized Gains	3,602	2,229
Gross Unrealized Losses	(1,371)	(72)
Fair Value	762,434	492,456
Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	398,217	533,853
Gross Unrealized Gains	2,871	3,968
Gross Unrealized Losses	(1,966)	(518)
Fair Value	399,122	537,303
U.S. agency securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	184,330	131,124
Gross Unrealized Gains	931	409
Gross Unrealized Losses	(490)	(11)
Fair Value	184,771	131,522
U.S. agency securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	154,059	63,645
Gross Unrealized Gains	758	612
Gross Unrealized Losses	(575)	(189)
Fair Value	154,242	64,068
Corporate bonds \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	87,137	67,169
Gross Unrealized Gains	296	580
Gross Unrealized Losses	(208)	(28)
Fair Value	87,225	67,721
Corporate bonds \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	148,437	141,307
Gross Unrealized Gains	1,158	1,832
Gross Unrealized Losses	(319)	(61)
Fair Value	149,276	143,078
Commercial paper \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,999
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	4,999
Municipal securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,965	6,667
Gross Unrealized Gains	67	109
Gross Unrealized Losses	(35)	0
Fair Value	9,997	6,776
Municipal securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,870	9,594
Gross Unrealized Gains	40	151
Gross Unrealized Losses	(91)	(39)
Fair Value	2,819	9,706
U.S. government securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	410,310	264,069
Gross Unrealized Gains	1,921	1,083
Gross Unrealized Losses	(571)	(17)
Fair Value	411,660	265,135
U.S. government securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	81,928	294,682
Gross Unrealized Gains	758	1,287
Gross Unrealized Losses	(897)	(190)
Fair Value	81,789	295,779
Foreign government securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	63,462	21,270
Gross Unrealized Gains	387	48
Gross Unrealized Losses	(67)	(16)
Fair Value	63,782	21,302
Foreign government securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,923	24,625
Gross Unrealized Gains	157	86
Gross Unrealized Losses	(84)	(39)
Fair Value	$ 10,996	$ 24,672

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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