INVESTMENTS IN DEBT SECURITIES - Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Short-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 295,788	$ 126,843
Less than 12 Months, Gross Unrealized Losses	(224)	(265)
Greater than 12 months, Fair Value	925	2,505
Greater than 12 months, Gross Unrealized Losses	(3)	(2)
Total, Fair Value	296,713	129,348
Total, Gross Unrealized Losses	(227)	(267)
Short-term debt securities \| U.S. agency securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	23,705	41,711
Less than 12 Months, Gross Unrealized Losses	(6)	(162)
Greater than 12 months, Fair Value	0	2,505
Greater than 12 months, Gross Unrealized Losses	0	(2)
Total, Fair Value	23,705	44,216
Total, Gross Unrealized Losses	(6)	(164)
Short-term debt securities \| Corporate bonds
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	166,579	15,255
Less than 12 Months, Gross Unrealized Losses	(73)	(14)
Greater than 12 months, Fair Value	925	0
Greater than 12 months, Gross Unrealized Losses	(3)	0
Total, Fair Value	167,504	15,255
Total, Gross Unrealized Losses	(76)	(14)
Short-term debt securities \| Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	750	2,566
Less than 12 Months, Gross Unrealized Losses	0	(3)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	750	2,566
Total, Gross Unrealized Losses	0	(3)
Short-term debt securities \| U.S. government securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	96,705	45,970
Less than 12 Months, Gross Unrealized Losses	(5)	(4)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	96,705	45,970
Total, Gross Unrealized Losses	(5)	(4)
Short-term debt securities \| Foreign government securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	8,049	21,341
Less than 12 Months, Gross Unrealized Losses	(140)	(82)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	8,049	21,341
Total, Gross Unrealized Losses	(140)	(82)
Long-term debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,146,440	49,182
Less than 12 Months, Gross Unrealized Losses	(1,462)	(57)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	1,146,440	49,182
Total, Gross Unrealized Losses	(1,462)	(57)
Long-term debt securities \| U.S. agency securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	124,811	1,406
Less than 12 Months, Gross Unrealized Losses	(175)	(3)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	124,811	1,406
Total, Gross Unrealized Losses	(175)	(3)
Long-term debt securities \| Corporate bonds
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	533,286	28,189
Less than 12 Months, Gross Unrealized Losses	(916)	(42)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	533,286	28,189
Total, Gross Unrealized Losses	(916)	(42)
Long-term debt securities \| Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	4,054
Less than 12 Months, Gross Unrealized Losses	(10)
Greater than 12 months, Fair Value	0
Greater than 12 months, Gross Unrealized Losses	0
Total, Fair Value	4,054
Total, Gross Unrealized Losses	(10)
Long-term debt securities \| U.S. government securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	470,202	8,658
Less than 12 Months, Gross Unrealized Losses	(337)	(2)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	470,202	8,658
Total, Gross Unrealized Losses	(337)	(2)
Long-term debt securities \| Foreign government securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	14,087	10,929
Less than 12 Months, Gross Unrealized Losses	(24)	(10)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Total, Fair Value	14,087	10,929
Total, Gross Unrealized Losses	$ (24)	$ (10)

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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