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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair Value on a Recurring Basis    
Total $ 156,382 $ 178,884
Money market funds    
Fair Value on a Recurring Basis    
Total 6,671 7,003
Government-sponsored enterprise securities    
Fair Value on a Recurring Basis    
Total 13,512  
Commercial paper    
Fair Value on a Recurring Basis    
Total 44,130 57,587
Corporate notes    
Fair Value on a Recurring Basis    
Total 91,680 113,709
Equity Investment    
Fair Value on a Recurring Basis    
Total 389 585
Level 1    
Fair Value on a Recurring Basis    
Total 6,671 7,003
Level 1 | Money market funds    
Fair Value on a Recurring Basis    
Total 6,671 7,003
Level 2    
Fair Value on a Recurring Basis    
Total 149,711 171,881
Level 2 | Government-sponsored enterprise securities    
Fair Value on a Recurring Basis    
Total 13,512  
Level 2 | Commercial paper    
Fair Value on a Recurring Basis    
Total 44,130 57,587
Level 2 | Corporate notes    
Fair Value on a Recurring Basis    
Total 91,680 113,709
Level 2 | Equity Investment    
Fair Value on a Recurring Basis    
Total $ 389 $ 585