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FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES - Assets - Recurring Basis (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair value of certain financial assets and liabilities        
Cash and cash equivalents $ 637,513 $ 528,622 $ 377,582 $ 2,175,417
Short-term investments 320,650 672,933    
Investments   2,366    
Asset transfers between Level 1 and Level 2 measurements 0 0    
Total fair value        
Fair value of certain financial assets and liabilities        
Cash 393,936 310,885    
Assets measured at fair value 957,671 1,202,437    
Cash and cash equivalents 637,513 528,622    
Short-term investments 320,650 672,933    
Accounts receivable, net 43 95    
Investments   2,366    
Accrued liabilities (535) (1,579)    
Foreign currency derivatives (492) (1,484)    
Total fair value | Money market funds        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 191,358 112,848    
Total fair value | Certificates of deposit        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 14,075 15,720    
Total fair value | Commercial paper        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 60,422 99,903    
Total fair value | Variable rate demand notes        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 4,005 49,954    
Total fair value | Municipal securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 177,118 529,984    
Total fair value | U.S. government agency securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 39,092 81,230    
Total fair value | U.S. Treasuries        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 78,157 3,397    
Level 1        
Fair value of certain financial assets and liabilities        
Cash 393,936 310,885    
Assets measured at fair value 585,294 423,733    
Cash and cash equivalents 585,294 423,733    
Level 1 | Money market funds        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 191,358 112,848    
Level 2        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 372,377 778,704    
Cash and cash equivalents 52,219 104,889    
Short-term investments 320,650 672,933    
Accounts receivable, net 43 95    
Investments   2,366    
Accrued liabilities (535) (1,579)    
Foreign currency derivatives (492) (1,484)    
Level 2 | Certificates of deposit        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 14,075 15,720    
Level 2 | Commercial paper        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 60,422 99,903    
Level 2 | Variable rate demand notes        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 4,005 49,954    
Level 2 | Municipal securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 177,118 529,984    
Level 2 | U.S. government agency securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 39,092 81,230    
Level 2 | U.S. Treasuries        
Fair value of certain financial assets and liabilities        
Assets measured at fair value $ 78,157 $ 3,397