CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Short-Term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 361,364	$ 304,380
Available-for-sale Securities, Gross Unrealized Gains	151	6
Available-for-sale Securities, Gross Unrealized Losses		(63)
Available-for-sale Securities, Fair Value Disclosure	361,515	304,323
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	800	500
Available-for-sale Securities, Gross Unrealized Losses		(1)
Available-for-sale Securities, Fair Value Disclosure	800	499
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		114,678
Available-for-sale Securities, Gross Unrealized Gains		1
Available-for-sale Securities, Gross Unrealized Losses		(19)
Available-for-sale Securities, Fair Value Disclosure		114,660
U.S. Government Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	360,564	189,202
Available-for-sale Securities, Gross Unrealized Gains	151	5
Available-for-sale Securities, Gross Unrealized Losses		(43)
Available-for-sale Securities, Fair Value Disclosure	$ 360,715	$ 189,164

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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