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CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Short-Term Investments (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis $ 509,029 $ 514,277
Available-for-sale Securities, Gross Unrealized Gains 1,430 190
Available-for-sale Securities, Gross Unrealized Losses (4) (6)
Available-for-sale Securities, Fair Value Disclosure 510,455 514,461
Certificates of Deposit [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 700 700
Available-for-sale Securities, Gross Unrealized Gains 2  
Available-for-sale Securities, Fair Value Disclosure 702 700
U.S. Government Bonds [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 508,329 513,577
Available-for-sale Securities, Gross Unrealized Gains 1,428 190
Available-for-sale Securities, Gross Unrealized Losses (4) (6)
Available-for-sale Securities, Fair Value Disclosure $ 509,753 $ 513,761