Marketable Securities - Summary Marketable Securities (Details) - USD ($) $ in Thousands	Jan. 31, 2021	Jan. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 2,003,560	$ 571,252
Gross Unrealized Gains	1,226	869
Gross Unrealized Losses	(376)	(61)
Estimated Fair Value	2,004,410	572,060
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	26,222	37,894
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	26,222	37,894
Agency bonds
Marketable Securities [Line Items]
Amortized Cost	461,335	141,157
Gross Unrealized Gains	79	49
Gross Unrealized Losses	(49)	(43)
Estimated Fair Value	461,365	141,163
Corporate and other debt securities
Marketable Securities [Line Items]
Amortized Cost	465,207	320,407
Gross Unrealized Gains	1,113	775
Gross Unrealized Losses	(64)	(16)
Estimated Fair Value	466,256	321,166
U.S. government agency securities
Marketable Securities [Line Items]
Amortized Cost	834,894	71,794
Gross Unrealized Gains	28	45
Gross Unrealized Losses	(257)	(2)
Estimated Fair Value	834,665	$ 71,837
Treasury bills
Marketable Securities [Line Items]
Amortized Cost	215,902
Gross Unrealized Gains	6
Gross Unrealized Losses	(6)
Estimated Fair Value	$ 215,902