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Fair Value Measurements - Assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Jan. 31, 2021	Jan. 31, 2020
Available-for-sale securities
Amortized Cost	$ 584,187	$ 818,970
Gross Unrealized Gains	388	1,204
Gross Unrealized Losses	(96)	(82)
Estimated Fair Value	584,479	820,092
Cash	281,700	75,800
Cash equivalents | Money market funds | Fair Value, Inputs, Level 1
Available-for-sale securities
Amortized Cost	284,312	165,424
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	284,312	165,424
Available-for-sale securities
Available-for-sale securities
Estimated Fair Value	300,167
Available-for-sale securities | Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	299,375	653,546
Gross Unrealized Gains	388	1,204
Gross Unrealized Losses	(96)	(82)
Estimated Fair Value	299,667	654,668
Available-for-sale securities | Commercial paper | Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	42,048	14,919
Gross Unrealized Gains	1	7
Gross Unrealized Losses	(23)	(1)
Estimated Fair Value	42,026	14,925
Available-for-sale securities | Corporate notes and bonds | Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	199,277	372,844
Gross Unrealized Gains	375	891
Gross Unrealized Losses	(67)	(31)
Estimated Fair Value	199,585	373,704
Available-for-sale securities | Corporate notes and bonds | Fair Value, Inputs, Level 3
Available-for-sale securities
Amortized Cost	500
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	500
Available-for-sale securities | U.S. Treasury securities | Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	4,998	90,697
Gross Unrealized Gains	0	153
Gross Unrealized Losses	0	(1)
Estimated Fair Value	4,998	90,849
Available-for-sale securities | U.S. government agency securities | Fair Value, Inputs, Level 2
Available-for-sale securities
Amortized Cost	53,052	175,086
Gross Unrealized Gains	12	153
Gross Unrealized Losses	(6)	(49)
Estimated Fair Value	$ 53,058	$ 175,190

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