INVESTMENTS IN DEBT SECURITIES - Short-Term and Long-Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,048,815
Fair Value	1,051,300
Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	520,017	$ 490,299
Gross Unrealized Gains	2,780	2,229
Gross Unrealized Losses	(957)	(72)
Fair Value	521,840	492,456
Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	528,798	533,853
Gross Unrealized Gains	3,934	3,968
Gross Unrealized Losses	(3,272)	(518)
Fair Value	529,460	537,303
U.S. agency securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	173,424	131,124
Gross Unrealized Gains	912	409
Gross Unrealized Losses	(208)	(11)
Fair Value	174,128	131,522
U.S. agency securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	166,075	63,645
Gross Unrealized Gains	704	612
Gross Unrealized Losses	(466)	(189)
Fair Value	166,313	64,068
Corporate bonds \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75,440	67,169
Gross Unrealized Gains	50	580
Gross Unrealized Losses	(627)	(28)
Fair Value	74,863	67,721
Corporate bonds \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	141,920	141,307
Gross Unrealized Gains	216	1,832
Gross Unrealized Losses	(2,161)	(61)
Fair Value	139,975	143,078
Commercial paper \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,996
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	2,996
Municipal securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,837	6,667
Gross Unrealized Gains	44	109
Gross Unrealized Losses	(5)	0
Fair Value	8,876	6,776
Municipal securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,464	9,594
Gross Unrealized Gains	32	151
Gross Unrealized Losses	(40)	(39)
Fair Value	8,456	9,706
U.S. government securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	238,012	264,069
Gross Unrealized Gains	1,722	1,083
Gross Unrealized Losses	(114)	(17)
Fair Value	239,620	265,135
U.S. government securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	177,308	294,682
Gross Unrealized Gains	2,648	1,287
Gross Unrealized Losses	(587)	(190)
Fair Value	179,369	295,779
Foreign securities \| Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,308	21,270
Gross Unrealized Gains	52	48
Gross Unrealized Losses	(3)	(16)
Fair Value	21,357	21,302
Foreign securities \| Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,031	24,625
Gross Unrealized Gains	334	86
Gross Unrealized Losses	(18)	(39)
Fair Value	$ 35,347	$ 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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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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