INVESTMENTS IN DEBT SECURITIES - Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 65,387	$ 386,696
Less than 12 Months, Gross Unrealized Losses	(60)	(296)
Greater than 12 months, Fair Value	22,482	126,525
Greater than 12 months, Gross Unrealized Losses	(12)	(726)
Total, Fair Value	87,869	513,221
Total, Gross Unrealized Losses	(72)	(1,022)
Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	93,046	176,388
Less than 12 Months, Gross Unrealized Losses	(490)	(899)
Greater than 12 months, Fair Value		58,275
Greater than 12 months, Gross Unrealized Losses	(28)	(327)
Total, Fair Value	93,046	234,663
Total, Gross Unrealized Losses	(518)	(1,226)
U.S. agency securities | Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	23,896	78,134
Less than 12 Months, Gross Unrealized Losses	(9)	(70)
Greater than 12 months, Fair Value	4,996	0
Greater than 12 months, Gross Unrealized Losses	(2)	0
Total, Fair Value	28,892	78,134
Total, Gross Unrealized Losses	(11)	(70)
U.S. agency securities | Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	16,740	20,504
Less than 12 Months, Gross Unrealized Losses	(189)	(29)
Greater than 12 months, Fair Value	0	10,133
Greater than 12 months, Gross Unrealized Losses	0	(49)
Total, Fair Value	16,740	30,637
Total, Gross Unrealized Losses	(189)	(78)
Corporate bonds | Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	5,507	38,052
Less than 12 Months, Gross Unrealized Losses	(27)	(61)
Greater than 12 months, Fair Value	2,502	62,479
Greater than 12 months, Gross Unrealized Losses	(1)	(307)
Total, Fair Value	8,009	100,531
Total, Gross Unrealized Losses	(28)	(368)
Corporate bonds | Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	16,708	119,333
Less than 12 Months, Gross Unrealized Losses	(61)	(824)
Greater than 12 months, Fair Value	0	20,306
Greater than 12 months, Gross Unrealized Losses	0	(126)
Total, Fair Value	16,708	139,639
Total, Gross Unrealized Losses	(61)	(950)
Municipal securities | Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,004	2,251
Less than 12 Months, Gross Unrealized Losses	0	(1)
Greater than 12 months, Fair Value	0	22,915
Greater than 12 months, Gross Unrealized Losses	0	(58)
Total, Fair Value	1,004	25,166
Total, Gross Unrealized Losses	0	(59)
Municipal securities | Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,005	9,701
Less than 12 Months, Gross Unrealized Losses	(39)	(14)
Greater than 12 months, Fair Value	0	3,260
Greater than 12 months, Gross Unrealized Losses	0	(12)
Total, Fair Value	1,005	12,961
Total, Gross Unrealized Losses	(39)	(26)
U.S. government securities | Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	21,481	240,979
Less than 12 Months, Gross Unrealized Losses	(8)	(148)
Greater than 12 months, Fair Value	14,984	41,131
Greater than 12 months, Gross Unrealized Losses	(9)	(361)
Total, Fair Value	36,465	282,110
Total, Gross Unrealized Losses	(17)	(509)
U.S. government securities | Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	42,210	25,850
Less than 12 Months, Gross Unrealized Losses	(162)	(32)
Greater than 12 months, Fair Value	0	24,576
Greater than 12 months, Gross Unrealized Losses	(28)	(140)
Total, Fair Value	42,210	50,426
Total, Gross Unrealized Losses	(190)	(172)
Foreign securities | Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	13,499	27,280
Less than 12 Months, Gross Unrealized Losses	(16)	(16)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	13,499	27,280
Total, Gross Unrealized Losses	(16)	(16)
Foreign securities | Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	16,383	1,000
Less than 12 Months, Gross Unrealized Losses	(39)	0
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	16,383	1,000
Total, Gross Unrealized Losses	$ (39)	$ 0