Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Oct. 31, 2020	Jan. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 1,140,108	$ 571,252
Gross Unrealized Gains	1,824	869
Gross Unrealized Losses	(507)	(61)
Estimated Fair Value	1,141,425	572,060
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	6,244	37,894
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	6,244	37,894
Agency bonds
Marketable Securities [Line Items]
Amortized Cost	409,695	141,157
Gross Unrealized Gains	75	49
Gross Unrealized Losses	(297)	(43)
Estimated Fair Value	409,473	141,163
Corporate and other debt securities
Marketable Securities [Line Items]
Amortized Cost	465,399	320,407
Gross Unrealized Gains	1,727	775
Gross Unrealized Losses	(180)	(16)
Estimated Fair Value	466,946	321,166
U.S. government agency securities
Marketable Securities [Line Items]
Amortized Cost	144,361	71,794
Gross Unrealized Gains	19	45
Gross Unrealized Losses	(26)	(2)
Estimated Fair Value	144,354	$ 71,837
Treasury bills
Marketable Securities [Line Items]
Amortized Cost	114,409
Gross Unrealized Gains	3
Gross Unrealized Losses	(4)
Estimated Fair Value	$ 114,408

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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