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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Jul. 31, 2019
Jan. 31, 2019
Cash equivalents    
Amortized Cost $ 224,290 $ 517,811
Available-for-sale securities    
Gross Unrealized Gains 1,115 400
Estimated Fair Value 706,109  
Total    
Amortized Cost 800,941 851,534
Gross Unrealized Losses (160) (142)
Estimated Fair Value 801,896 851,792
Level 2    
Available-for-sale securities    
Amortized Cost 705,154 501,471
Gross Unrealized Gains 1,115 400
Gross Unrealized Losses (160) (142)
Estimated Fair Value 706,109 501,729
Commercial paper | Level 2    
Available-for-sale securities    
Amortized Cost 41,197 86,655
Gross Unrealized Gains 45 4
Gross Unrealized Losses (2) (21)
Estimated Fair Value 41,240 86,638
Corporate notes and bonds | Level 2    
Available-for-sale securities    
Amortized Cost 351,691 287,496
Gross Unrealized Gains 835 389
Gross Unrealized Losses (68) (105)
Estimated Fair Value 352,458 287,780
U.S. Treasury securities | Level 2    
Available-for-sale securities    
Amortized Cost 174,897 4,982
Gross Unrealized Gains 172 0
Gross Unrealized Losses (31) (1)
Estimated Fair Value 175,038 4,981
U.S. government agency securities | Level 2    
Available-for-sale securities    
Amortized Cost 137,369 36,021
Gross Unrealized Gains 63 7
Gross Unrealized Losses (59) (4)
Estimated Fair Value 137,373 36,024
Money market funds | Level 1    
Cash equivalents    
Amortized Cost 95,787 350,063
Estimated Fair Value $ 95,787 350,063
Commercial paper | Level 2    
Cash equivalents    
Amortized Cost   76,828
Gross Unrealized Losses   (11)
Estimated Fair Value   76,817
Corporate notes and bonds | Level 2    
Cash equivalents    
Amortized Cost   2,998
Estimated Fair Value   2,998
U.S. government agency securities | Level 2    
Cash equivalents    
Amortized Cost   6,491
Estimated Fair Value   $ 6,491