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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Apr. 30, 2019
Jan. 31, 2019
Cash equivalents    
Amortized Cost $ 236,476 $ 517,811
Available-for-sale securities    
Gross Unrealized Gains 657 400
Estimated Fair Value 700,331  
Total    
Amortized Cost 796,064 851,534
Gross Unrealized Losses (62) (142)
Estimated Fair Value 796,659 851,792
Level 2    
Available-for-sale securities    
Amortized Cost 704,734 501,471
Gross Unrealized Gains 657 400
Gross Unrealized Losses (62) (142)
Estimated Fair Value 705,329 501,729
Commercial paper | Level 2    
Available-for-sale securities    
Amortized Cost 62,695 86,655
Gross Unrealized Gains 31 4
Gross Unrealized Losses (8) (21)
Estimated Fair Value 62,718 86,638
Corporate notes and bonds | Level 2    
Available-for-sale securities    
Amortized Cost 334,912 287,496
Gross Unrealized Gains 582 389
Gross Unrealized Losses (34) (105)
Estimated Fair Value 335,460 287,780
U.S. Treasury securities | Level 2    
Available-for-sale securities    
Amortized Cost 232,152 4,982
Gross Unrealized Gains 32 0
Gross Unrealized Losses (15) (1)
Estimated Fair Value 232,169 4,981
U.S. government agency securities | Level 2    
Available-for-sale securities    
Amortized Cost 69,977 36,021
Gross Unrealized Gains 12 7
Gross Unrealized Losses (5) (4)
Estimated Fair Value 69,984 36,024
Money market funds | Level 1    
Cash equivalents    
Amortized Cost 91,330 350,063
Estimated Fair Value 91,330 350,063
Commercial paper | Level 2    
Cash equivalents    
Amortized Cost 4,998 76,828
Gross Unrealized Losses 0 (11)
Estimated Fair Value $ 4,998 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