Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Jul. 31, 2020	Jan. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 730,223	$ 571,252
Gross Unrealized Gains	2,845	869
Gross Unrealized Losses	(73)	(61)
Estimated Fair Value	732,995	572,060
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	6,990	37,894
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	6,990	37,894
Agency bonds
Marketable Securities [Line Items]
Amortized Cost	302,221	141,157
Gross Unrealized Gains	193	49
Gross Unrealized Losses	(45)	(43)
Estimated Fair Value	302,369	141,163
Corporate and other debt securities
Marketable Securities [Line Items]
Amortized Cost	362,539	320,407
Gross Unrealized Gains	2,571	775
Gross Unrealized Losses	(28)	(16)
Estimated Fair Value	365,082	321,166
U.S. government agency securities
Marketable Securities [Line Items]
Amortized Cost	43,479	71,794
Gross Unrealized Gains	81	45
Gross Unrealized Losses	0	(2)
Estimated Fair Value	43,560	$ 71,837
Treasury bills
Marketable Securities [Line Items]
Amortized Cost	14,994
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	$ 14,994

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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