Cash Equivalents and Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Oct. 31, 2019	Jan. 31, 2019
Marketable Securities [Line Items]
Cash equivalents, Amortized Cost	$ 99,494	$ 78
Cash equivalents, Estimated Fair Value	99,494	78
Amortized Cost	579,875	112,912
Gross Unrealized Gains	774	0
Gross Unrealized Losses	(123)	(135)
Estimated Fair Value	580,526	112,777
Money market funds
Marketable Securities [Line Items]
Cash equivalents, Amortized Cost	99,494	78
Cash equivalents, Estimated Fair Value	99,494	78
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	48,841	1,243
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	48,841	1,243
Corporate bonds
Marketable Securities [Line Items]
Amortized Cost		53,267
Gross Unrealized Gains		0
Gross Unrealized Losses		(53)
Estimated Fair Value		53,214
Agency bonds
Marketable Securities [Line Items]
Amortized Cost	78,963	32,675
Gross Unrealized Gains	12	0
Gross Unrealized Losses	(84)	(71)
Estimated Fair Value	78,891	32,604
Corporate and other debt securities
Marketable Securities [Line Items]
Amortized Cost	296,419
Gross Unrealized Gains	597
Gross Unrealized Losses	(37)
Estimated Fair Value	296,979
U.S. government agency securities
Marketable Securities [Line Items]
Amortized Cost	106,224	24,028
Gross Unrealized Gains	143	0
Gross Unrealized Losses	(2)	(11)
Estimated Fair Value	106,365	24,017
Treasury bills
Marketable Securities [Line Items]
Amortized Cost	49,428	1,699
Gross Unrealized Gains	22	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 49,450	$ 1,699

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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Amount of 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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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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