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Fair Value Measurements (Information about Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Detail) - USD ($)
$ in Thousands
Jan. 31, 2019
Jan. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities $ 1,312,007 $ 2,551,107
Marketable equity investments 44,127 0
Foreign currency derivative assets 22,570  
Foreign currency derivative liabilities 3,135  
Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity investments 44,127  
Foreign currency derivative assets 22,570 98
Total assets 1,615,775 3,103,009
Foreign currency derivative liabilities 3,135 32,912
Total liabilities 3,135 32,912
Recurring | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 396,230 796,835
Recurring | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 241,836 682,424
Recurring | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 419,768 469,121
Recurring | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 254,173 602,727
Recurring | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 237,071 551,804
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 677,428 1,348,639
Foreign currency derivative liabilities 0 0
Total liabilities 0 0
Recurring | Level 1 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 396,230 796,835
Recurring | Level 1 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 1 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 1 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 1 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 237,071 551,804
Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity investments 0  
Foreign currency derivative assets 22,570 98
Total assets 938,347 1,754,370
Foreign currency derivative liabilities 3,135 32,912
Total liabilities 3,135 32,912
Recurring | Level 2 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 2 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 241,836 682,424
Recurring | Level 2 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 419,768 469,121
Recurring | Level 2 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 254,173 602,727
Recurring | Level 2 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
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
Recurring | Level 3 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 3 | U.S. agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 3 | Corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 3 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 0 0
Recurring | Level 3 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities $ 0 $ 0