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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Assets    
Total $ 119,366 $ 127,182
Money market funds    
Assets    
Total 9,946 11,193
Government-sponsored enterprise securities    
Assets    
Total 17,462 17,455
Commercial paper    
Assets    
Total 20,595 31,069
Corporate notes    
Assets    
Total 71,363 67,465
Level 1    
Assets    
Total 9,946 11,193
Level 1 | Money market funds    
Assets    
Total 9,946 11,193
Level 2    
Assets    
Total 109,420 115,989
Level 2 | Government-sponsored enterprise securities    
Assets    
Total 17,462 17,455
Level 2 | Commercial paper    
Assets    
Total 20,595 31,069
Level 2 | Corporate notes    
Assets    
Total $ 71,363 $ 67,465