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FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES - Assets - Recurring Basis (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Fair value of certain financial assets and liabilities        
Cash and cash equivalents $ 511,360 $ 528,622 $ 576,337 $ 377,582
Short-term investments 585,222 672,933    
Investments 3,497 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 and cash equivalents 511,360 528,622    
Short-term investments 585,222 672,933    
Accounts receivable, net 208 95    
Investments 3,497 2,366    
Accrued liabilities (303) (1,579)    
Assets measured at fair value 1,099,984 1,202,437    
Cash 394,221 310,885    
Foreign currency derivatives (95) (1,484)    
Money market funds | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 39,228 112,848    
Certificates of deposit | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 16,985 15,720    
Commercial paper | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 60,825 99,903    
Variable rate demand notes | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 27,416 49,954    
Municipal securities | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 481,250 529,984    
U.S. government agency securities | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 80,154 81,230    
U.S. Treasuries | Total fair value        
Fair value of certain financial assets and liabilities        
Assets measured at fair value   3,397    
Level 1        
Fair value of certain financial assets and liabilities        
Cash and cash equivalents 433,449 423,733    
Assets measured at fair value 433,449 423,733    
Cash 394,221 310,885    
Level 1 | Money market funds        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 39,228 112,848    
Level 2        
Fair value of certain financial assets and liabilities        
Cash and cash equivalents 77,911 104,889    
Short-term investments 585,222 672,933    
Accounts receivable, net 208 95    
Investments 3,497 2,366    
Accrued liabilities (303) (1,579)    
Assets measured at fair value 666,535 778,704    
Foreign currency derivatives (95) (1,484)    
Level 2 | Certificates of deposit        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 16,985 15,720    
Level 2 | Commercial paper        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 60,825 99,903    
Level 2 | Variable rate demand notes        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 27,416 49,954    
Level 2 | Municipal securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value 481,250 529,984    
Level 2 | U.S. government agency securities        
Fair value of certain financial assets and liabilities        
Assets measured at fair value $ 80,154 81,230    
Level 2 | U.S. Treasuries        
Fair value of certain financial assets and liabilities        
Assets measured at fair value   $ 3,397