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Fair Value Measurements - Information about Assets that are Measured at Fair Value on a Recurring Basis (Detail) - USD ($)
$ in Thousands
Jul. 31, 2019
Jan. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities $ 1,388,012 $ 1,312,007
Marketable equity investments 0 44,127
Foreign currency derivative assets 53,135  
Foreign currency derivative liabilities 518  
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity investments   44,127
Foreign currency derivative assets 53,135 22,570
Total assets 1,789,256 1,615,775
Foreign currency derivative liabilities 518 3,135
Total liabilities 518 3,135
Fair Value, Measurements, Recurring | U.S. treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 524,985 396,230
Fair Value, Measurements, Recurring | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 240,218 241,836
Fair Value, Measurements, Recurring | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 439,923 419,768
Fair Value, Measurements, Recurring | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 182,886 254,173
Fair Value, Measurements, Recurring | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 348,109 237,071
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity investments   44,127
Foreign currency derivative assets 0 0
Total assets 873,094 677,428
Foreign currency derivative liabilities 0 0
Total liabilities 0 0
Fair Value, Measurements, Recurring | Level 1 | U.S. treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 524,985 396,230
Fair Value, Measurements, Recurring | Level 1 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 1 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 1 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 348,109 237,071
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity investments   0
Foreign currency derivative assets 53,135 22,570
Total assets 916,162 938,347
Foreign currency derivative liabilities 518 3,135
Total liabilities 518 3,135
Fair Value, Measurements, Recurring | Level 2 | U.S. treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 2 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 240,218 241,836
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 439,923 419,768
Fair Value, Measurements, Recurring | Level 2 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 182,886 254,173
Fair Value, Measurements, Recurring | Level 2 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity investments   0
Foreign currency derivative assets 0 0
Total assets 0 0
Foreign currency derivative liabilities 0 0
Total liabilities 0 0
Fair Value, Measurements, Recurring | Level 3 | U.S. treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Fair Value, Measurements, Recurring | Level 3 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities $ 0 $ 0