INVESTMENTS IN DEBT SECURITIES - Short-term and Long-term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,024,152
Fair Value	1,029,759
Short-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	490,299	$ 541,684
Gross Unrealized Gains	2,229	329
Gross Unrealized Losses	(72)	(1,022)
Fair Value	492,456	540,991
Short-term debt securities \| U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	131,124	80,160
Gross Unrealized Gains	409	32
Gross Unrealized Losses	(11)	(70)
Fair Value	131,522	80,122
Short-term debt securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	67,169	109,807
Gross Unrealized Gains	580	80
Gross Unrealized Losses	(28)	(368)
Fair Value	67,721	109,519
Short-term debt securities \| Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,667	27,839
Gross Unrealized Gains	109	52
Gross Unrealized Losses	0	(59)
Fair Value	6,776	27,832
Short-term debt securities \| U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	264,069	292,615
Gross Unrealized Gains	1,083	161
Gross Unrealized Losses	(17)	(509)
Fair Value	265,135	292,267
Short-term debt securities \| Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,270	31,263
Gross Unrealized Gains	48	4
Gross Unrealized Losses	(16)	(16)
Fair Value	21,302	31,251
Long-term debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	533,853	464,545
Gross Unrealized Gains	3,968	1,361
Gross Unrealized Losses	(518)	(1,226)
Fair Value	537,303	464,680
Long-term debt securities \| U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	63,645	114,444
Gross Unrealized Gains	612	194
Gross Unrealized Losses	(189)	(78)
Fair Value	64,068	114,560
Long-term debt securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	141,307	159,783
Gross Unrealized Gains	1,832	419
Gross Unrealized Losses	(61)	(950)
Fair Value	143,078	159,252
Long-term debt securities \| Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,594	28,453
Gross Unrealized Gains	151	167
Gross Unrealized Losses	(39)	(26)
Fair Value	9,706	28,594
Long-term debt securities \| U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	294,682	153,743
Gross Unrealized Gains	1,287	553
Gross Unrealized Losses	(190)	(172)
Fair Value	295,779	154,124
Long-term debt securities \| Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,625	8,122
Gross Unrealized Gains	86	28
Gross Unrealized Losses	(39)	0
Fair Value	$ 24,672	$ 8,150

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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