DEBT SECURITIES - Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	$ 198,170	$ 208,906
Debt securities available for sale, unrealized gain, current	621	598
Debt securities available for sale unrealized loss, current	(6)	(10)
Debt securities, available-for-sale, current	198,785	209,494
Debt securities, available-for-sale, amortized cost basis, noncurrent	56,485	126,300
Debt securities, available for sale unrealized gain, noncurrent	523	351
Debt securities available for sale unrealized gain loss, noncurrent	(7)	(57)
Debt securities, available-for-sale, noncurrent	57,001	126,594
Debt securities, available-for-sale, amortized cost	254,655	335,206
Debt securities, available for sale, unrealized gains	1,144	949
Debt securities, available-for-sale, unrealized gain (loss)	(13)	(67)
Aggregate Estimated Fair Value	255,786	336,088
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	63,167	17,136
Debt securities available for sale, unrealized gain, current	2	16
Debt securities available for sale unrealized loss, current	(6)	0
Debt securities, available-for-sale, current	63,163	17,152
Aggregate Estimated Fair Value	63,163	17,152
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	91,922	191,770
Debt securities available for sale, unrealized gain, current	619	582
Debt securities available for sale unrealized loss, current	0	(10)
Debt securities, available-for-sale, current	92,541	192,342
Debt securities, available-for-sale, amortized cost basis, noncurrent	46,485	113,799
Debt securities, available for sale unrealized gain, noncurrent	522	351
Debt securities available for sale unrealized gain loss, noncurrent	0	(56)
Debt securities, available-for-sale, noncurrent	47,007	114,094
Aggregate Estimated Fair Value	139,548	306,436
Securities of government sponsored entities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	43,081
Debt securities available for sale, unrealized gain, current	0
Debt securities available for sale unrealized loss, current	0
Debt securities, available-for-sale, current	43,081
Debt securities, available-for-sale, amortized cost basis, noncurrent	10,000	12,501
Debt securities, available for sale unrealized gain, noncurrent	1	0
Debt securities available for sale unrealized gain loss, noncurrent	(7)	(1)
Debt securities, available-for-sale, noncurrent	9,994	12,500
Aggregate Estimated Fair Value	$ 53,075	$ 12,500

Debt Securities Available For Sale, Unrealized Gain, Current

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Debt Securities, Available For Sale Unrealized Gain, Noncurrent

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Debt Securities, Available-for-sale, Amortized Cost Basis, Current

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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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Amortized cost of 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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