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Cash Equivalents and Short-term Investments Schedule of Amortized Costs, Unrealized Gains and Losses and Estimated Fair Value of Cash Equivalents and Short-term Investments (Details) - USD ($)
$ in Thousands
Jan. 31, 2020
Jan. 31, 2019
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost $ 881,779 $ 265,396
Unrealized Gain 1,205 50
Unrealized Loss (8) (72)
Estimated Fair Value 882,976 265,374
Total Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost 1,318,359 516,231
Unrealized Gain 1,205 50
Unrealized Loss (8) (73)
Estimated Fair Value 1,319,556 516,208
U.S. treasury securities    
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost 575,920 195,913
Unrealized Gain 686 37
Unrealized Loss (8) (53)
Estimated Fair Value 576,598 195,897
Corporate debt securities    
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost 305,859 69,483
Unrealized Gain 519 13
Unrealized Loss 0 (19)
Estimated Fair Value 306,378 69,477
Cash and Cash Equivalents    
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost 436,580 250,835
Unrealized Gain 0 0
Unrealized Loss 0 (1)
Estimated Fair Value 436,580 250,834
Cash and Cash Equivalents | Money market funds    
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost 416,584 247,426
Unrealized Gain 0 0
Unrealized Loss 0 0
Estimated Fair Value 416,584 247,426
Cash and Cash Equivalents | U.S. treasury securities    
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost 19,996  
Unrealized Gain 0  
Unrealized Loss 0  
Estimated Fair Value $ 19,996  
Cash and Cash Equivalents | Corporate debt securities    
Cash Equivalents And Short Term Investments [Abstract]    
Amortized Cost   3,409
Unrealized Gain   0
Unrealized Loss   (1)
Estimated Fair Value   $ 3,408