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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value on a Recurring Basis    
Total $ 182,681 $ 107,613
Money market funds    
Fair Value on a Recurring Basis    
Total 10,065 11,030
Commercial paper    
Fair Value on a Recurring Basis    
Total 63,059 13,173
Corporate notes    
Fair Value on a Recurring Basis    
Total 108,706 70,950
Equity Investment    
Fair Value on a Recurring Basis    
Total 851  
US Government-sponsored Enterprises Debt Securities    
Fair Value on a Recurring Basis    
Total   12,460
Level 1    
Fair Value on a Recurring Basis    
Total 10,065 11,030
Level 1 | Money market funds    
Fair Value on a Recurring Basis    
Total 10,065 11,030
Level 2    
Fair Value on a Recurring Basis    
Total 172,616 96,583
Level 2 | Commercial paper    
Fair Value on a Recurring Basis    
Total 63,059 13,173
Level 2 | Corporate notes    
Fair Value on a Recurring Basis    
Total 108,706 70,950
Level 2 | Equity Investment    
Fair Value on a Recurring Basis    
Total $ 851  
Level 2 | US Government-sponsored Enterprises Debt Securities    
Fair Value on a Recurring Basis    
Total   $ 12,460