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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value on a Recurring Basis    
Total $ 178,884 $ 107,613
Money market funds    
Fair Value on a Recurring Basis    
Total 7,003 11,030
Commercial paper    
Fair Value on a Recurring Basis    
Total 57,587 13,173
Corporate notes    
Fair Value on a Recurring Basis    
Total 113,709 70,950
Equity Investment    
Fair Value on a Recurring Basis    
Total 585  
US Government-sponsored Enterprises Debt Securities    
Fair Value on a Recurring Basis    
Total   12,460
Level 1    
Fair Value on a Recurring Basis    
Total 7,003 11,030
Level 1 | Money market funds    
Fair Value on a Recurring Basis    
Total 7,003 11,030
Level 2    
Fair Value on a Recurring Basis    
Total 171,881 96,583
Level 2 | Commercial paper    
Fair Value on a Recurring Basis    
Total 57,587 13,173
Level 2 | Corporate notes    
Fair Value on a Recurring Basis    
Total 113,709 70,950
Level 2 | Equity Investment    
Fair Value on a Recurring Basis    
Total $ 585  
Level 2 | US Government-sponsored Enterprises Debt Securities    
Fair Value on a Recurring Basis    
Total   $ 12,460