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FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES - Assets - Recurring Basis (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Fair value of certain financial assets and liabilities        
Cash and cash equivalents $ 618,344 $ 637,513 $ 511,360 $ 528,622
Short-term investments 263,697 320,650    
Asset transfers between Level 1 and Level 2 measurements 0      
Total fair value        
Fair value of certain financial assets and liabilities        
Cash 444,489 393,936    
Assets measured at fair value 881,908 957,671    
Foreign currency derivatives (133)      
Cash and cash equivalents 618,344 637,513    
Short-term investments 263,697 320,650    
Accounts receivable, net 181 43    
Accrued liabilities (314) (535)    
Foreign currency derivatives   (492)    
Total fair value | Money market funds        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 79,611 191,358    
Total fair value | Certificates of deposit        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 21,929 14,075    
Total fair value | Commercial paper        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 63,920 60,422    
Total fair value | Variable rate demand notes        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 6,806 4,005    
Total fair value | Municipal securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 117,521 177,118    
Total fair value | U.S. government agency securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 50,505 39,092    
Total fair value | U.S. treasuries        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 97,260 78,157    
Level 1        
Fair value of certain financial assets and liabilities        
Cash 444,489 393,936    
Assets measured at fair value 524,100 585,294    
Cash and cash equivalents 524,100 585,294    
Level 1 | Money market funds        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 79,611 191,358    
Level 2        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 357,808 372,377    
Foreign currency derivatives (133)      
Cash and cash equivalents 94,244 52,219    
Short-term investments 263,697 320,650    
Accounts receivable, net 181 43    
Accrued liabilities (314) (535)    
Foreign currency derivatives   (492)    
Level 2 | Certificates of deposit        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 21,929 14,075    
Level 2 | Commercial paper        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 63,920 60,422    
Level 2 | Variable rate demand notes        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 6,806 4,005    
Level 2 | Municipal securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 117,521 177,118    
Level 2 | U.S. government agency securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 50,505 39,092    
Level 2 | U.S. treasuries        
Fair value of certain financial assets and liabilities        
Assets measured at fair value $ 97,260 $ 78,157