CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Short-Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 414,473	$ 514,277
Available-for-sale Securities, Gross Unrealized Gains	2,559	190
Available-for-sale Securities, Gross Unrealized Losses		(6)
Available-for-sale Securities, Fair Value Disclosure	417,032	514,461
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	750	700
Available-for-sale Securities, Gross Unrealized Gains	2
Available-for-sale Securities, Fair Value Disclosure	752	700
U.S. Government Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	413,723	513,577
Available-for-sale Securities, Gross Unrealized Gains	2,557	190
Available-for-sale Securities, Gross Unrealized Losses		(6)
Available-for-sale Securities, Fair Value Disclosure	$ 416,280	$ 513,761

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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