Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Apr. 30, 2020	Jan. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 614,827	$ 571,252
Gross Unrealized Gains	2,287	869
Gross Unrealized Losses	(400)	(61)
Estimated Fair Value	616,714	572,060
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	11,892	37,894
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	11,892	37,894
Agency bonds
Marketable Securities [Line Items]
Amortized Cost	157,493	141,157
Gross Unrealized Gains	285	49
Gross Unrealized Losses	(56)	(43)
Estimated Fair Value	157,722	141,163
Corporate and other debt securities
Marketable Securities [Line Items]
Amortized Cost	392,575	320,407
Gross Unrealized Gains	1,780	775
Gross Unrealized Losses	(344)	(16)
Estimated Fair Value	394,011	321,166
U.S. government agency securities
Marketable Securities [Line Items]
Amortized Cost	49,871	71,794
Gross Unrealized Gains	218	45
Gross Unrealized Losses	0	(2)
Estimated Fair Value	50,089	$ 71,837
Treasury bills
Marketable Securities [Line Items]
Amortized Cost	2,996
Gross Unrealized Gains	4
Gross Unrealized Losses	0
Estimated Fair Value	$ 3,000

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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