DEBT SECURITIES - Available for Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	$ 132,352	$ 208,906
Debt securities available for sale, unrealized gain, current	770	598
Debt securities available for sale unrealized loss, current	0	(10)
Debt securities, available-for-sale, current	133,122	209,494
Debt securities, available-for-sale, amortized cost basis, noncurrent	86,244	126,300
Debt securities, available for sale unrealized gain, noncurrent	849	351
Debt securities available for sale unrealized gain loss, noncurrent	(9)	(57)
Debt securities, available-for-sale, noncurrent	87,084	126,594
Debt securities, available-for-sale, amortized cost	218,596	335,206
Debt securities, available for sale, unrealized gains	1,619	949
Debt securities, available-for-sale, unrealized gain (loss)	(9)	(67)
Aggregate Estimated Fair Value	220,206	336,088
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	0	17,136
Debt securities available for sale, unrealized gain, current	0	16
Debt securities available for sale unrealized loss, current	0	0
Debt securities, available-for-sale, current	0	17,152
Aggregate Estimated Fair Value	0	17,152
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, amortized cost basis, current	132,352	191,770
Debt securities available for sale, unrealized gain, current	770	582
Debt securities available for sale unrealized loss, current	0	(10)
Debt securities, available-for-sale, current	133,122	192,342
Debt securities, available-for-sale, amortized cost basis, noncurrent	76,244	113,799
Debt securities, available for sale unrealized gain, noncurrent	848	351
Debt securities available for sale unrealized gain loss, noncurrent	(9)	(56)
Debt securities, available-for-sale, noncurrent	77,083	114,094
Aggregate Estimated Fair Value	210,205	306,436
Securities of government sponsored entities
Debt Securities, Available-for-sale [Line Items]
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	0	(1)
Debt securities, available-for-sale, noncurrent	10,001	12,500
Aggregate Estimated Fair Value	$ 10,001	$ 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, Unrealized Loss, Current

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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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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