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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair Value on a Recurring Basis    
Total $ 114,816 $ 127,182
Money market funds    
Fair Value on a Recurring Basis    
Total 4,501 11,193
Government-sponsored enterprise securities    
Fair Value on a Recurring Basis    
Total 17,453 17,455
Commercial paper    
Fair Value on a Recurring Basis    
Total 19,439 31,069
Corporate notes    
Fair Value on a Recurring Basis    
Total 73,423 67,465
Level 1    
Fair Value on a Recurring Basis    
Total 4,501 11,193
Level 1 | Money market funds    
Fair Value on a Recurring Basis    
Total 4,501 11,193
Level 2    
Fair Value on a Recurring Basis    
Total 110,315 115,989
Level 2 | Government-sponsored enterprise securities    
Fair Value on a Recurring Basis    
Total 17,453 17,455
Level 2 | Commercial paper    
Fair Value on a Recurring Basis    
Total 19,439 31,069
Level 2 | Corporate notes    
Fair Value on a Recurring Basis    
Total $ 73,423 $ 67,465