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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Assets    
Total $ 133,709 $ 144,361
Money market funds    
Assets    
Total 4,284 4,577
Government-sponsored enterprise securities    
Assets    
Total 17,011 11,949
Commercial paper    
Assets    
Total 40,059 35,307
Corporate notes    
Assets    
Total 72,355 92,528
Level 1    
Assets    
Total 4,284 4,577
Level 1 | Money market funds    
Assets    
Total 4,284 4,577
Level 2    
Assets    
Total 129,425 139,784
Level 2 | Government-sponsored enterprise securities    
Assets    
Total 17,011 11,949
Level 2 | Commercial paper    
Assets    
Total 40,059 35,307
Level 2 | Corporate notes    
Assets    
Total $ 72,355 $ 92,528