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FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES - Assets - Recurring Basis (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fair value of certain financial assets and liabilities    
Cash and cash equivalents $ 659,687 $ 528,622
Short-term investments 211,093 672,933
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 and cash equivalents 659,687 528,622
Short-term investments 211,093 672,933
Accounts receivable, net 21 95
Investments   2,366
Accrued liabilities (1,017) (1,579)
Assets measured at fair value 869,784 1,202,437
Cash 358,788 310,885
Foreign currency derivatives (996) (1,484)
Money market funds | Total fair value    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 299,949 112,848
Certificates of deposit | Total fair value    
Fair value of certain financial assets and liabilities    
Assets measured at fair value   15,720
Commercial paper | Total fair value    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 16,127 99,903
Variable rate demand notes | Total fair value    
Fair value of certain financial assets and liabilities    
Assets measured at fair value   49,954
Municipal securities | Total fair value    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 148,997 529,984
U.S. government agency securities | Total fair value    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 46,919 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 658,737 423,733
Assets measured at fair value 658,737 423,733
Cash 358,788 310,885
Level 1 | Money market funds    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 299,949 112,848
Level 2    
Fair value of certain financial assets and liabilities    
Cash and cash equivalents 950 104,889
Short-term investments 211,093 672,933
Accounts receivable, net 21 95
Investments   2,366
Accrued liabilities (1,017) (1,579)
Assets measured at fair value 211,047 778,704
Foreign currency derivatives (996) (1,484)
Level 2 | Certificates of deposit    
Fair value of certain financial assets and liabilities    
Assets measured at fair value   15,720
Level 2 | Commercial paper    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 16,127 99,903
Level 2 | Variable rate demand notes    
Fair value of certain financial assets and liabilities    
Assets measured at fair value   49,954
Level 2 | Municipal securities    
Fair value of certain financial assets and liabilities    
Assets measured at fair value 148,997 529,984
Level 2 | U.S. government agency securities    
Fair value of certain financial assets and liabilities    
Assets measured at fair value $ 46,919 81,230
Level 2 | U.S. Treasuries    
Fair value of certain financial assets and liabilities    
Assets measured at fair value   $ 3,397