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FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES - Assets - Recurring Basis (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair value of certain assets        
Cash and cash equivalents $ 528,622 $ 377,582 $ 2,175,417 $ 370,323
Short-term investments 672,933 220,554    
Investments 2,366 2,394    
Asset transfers between Level 1 and Level 2 measurements 0 0    
Total fair value        
Fair value of certain assets        
Cash and cash equivalents 528,622 377,582    
Short-term investments 672,933 220,554    
Accounts receivable, net 95 236    
Investments 2,366 2,394    
Accrued liabilities (1,579) (764)    
Assets measured at fair value 1,202,437 600,002    
Cash 310,885 278,972    
Foreign currency derivatives (1,484) (528)    
Money market funds | Total fair value        
Fair value of certain assets        
Assets measured at fair value 112,848 76,112    
Certificates of deposit | Total fair value        
Fair value of certain assets        
Assets measured at fair value 15,720      
Commercial paper | Total fair value        
Fair value of certain assets        
Assets measured at fair value 99,903 47,855    
Variable rate demand notes | Total fair value        
Fair value of certain assets        
Assets measured at fair value 49,954 13,923    
Municipal securities | Total fair value        
Fair value of certain assets        
Assets measured at fair value 529,984 157,617    
U.S. government agency securities | Total fair value        
Fair value of certain assets        
Assets measured at fair value 81,230 26,051    
U.S. Treasuries | Total fair value        
Fair value of certain assets        
Assets measured at fair value 3,397      
Level 1        
Fair value of certain assets        
Cash and cash equivalents 423,733 355,084    
Assets measured at fair value 423,733 355,084    
Cash 310,885 278,972    
Level 1 | Money market funds        
Fair value of certain assets        
Assets measured at fair value 112,848 76,112    
Level 2        
Fair value of certain assets        
Cash and cash equivalents 104,889 22,498    
Short-term investments 672,933 220,554    
Accounts receivable, net 95 236    
Investments 2,366 2,394    
Accrued liabilities (1,579) (764)    
Assets measured at fair value 778,704 244,918    
Foreign currency derivatives (1,484) (528)    
Level 2 | Certificates of deposit        
Fair value of certain assets        
Assets measured at fair value 15,720      
Level 2 | Commercial paper        
Fair value of certain assets        
Assets measured at fair value 99,903 47,855    
Level 2 | Variable rate demand notes        
Fair value of certain assets        
Assets measured at fair value 49,954 13,923    
Level 2 | Municipal securities        
Fair value of certain assets        
Assets measured at fair value 529,984 157,617    
Level 2 | U.S. government agency securities        
Fair value of certain assets        
Assets measured at fair value 81,230 $ 26,051    
Level 2 | U.S. Treasuries        
Fair value of certain assets        
Assets measured at fair value $ 3,397