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FINANCIAL INSTRUMENTS - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Assets:    
Cash equivalents   $ 195,902
Marketable securities $ 34,865  
Total 301,521  
Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents   189,207
Total 158,648  
Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents   6,695
Total 142,873  
Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents   0
Total 0  
Money market funds    
Assets:    
Cash equivalents 158,648 189,207
Money market funds | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 158,648 189,207
Money market funds | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 0 0
Money market funds | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents 0 0
Commercial paper    
Assets:    
Cash equivalents 48,048  
Commercial paper | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 0  
Commercial paper | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 48,048  
Commercial paper | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents 0  
Treasury discount notes    
Assets:    
Cash equivalents 44,960 500
Marketable securities 34,865  
Treasury discount notes | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 0 0
Marketable securities 0  
Treasury discount notes | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 44,960 500
Marketable securities 34,865  
Treasury discount notes | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents 0 0
Marketable securities 0  
Time deposits    
Assets:    
Cash equivalents 15,000  
Time deposits | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 0  
Time deposits | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 15,000  
Time deposits | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents $ 0  
Certificates of deposit    
Assets:    
Cash equivalents   6,195
Certificates of deposit | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents   0
Certificates of deposit | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents   6,195
Certificates of deposit | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents   $ 0