Financial Instruments - Summary of Cash Equivalents, Marketable Securities and Restricted Cash by Significant Investment Categories (Details) - USD ($) $ in Thousands	Oct. 31, 2018	Jan. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 805,503	$ 365,378
Gross Unrealized Gains	32	116
Gross Unrealized Losses	(3,131)	(2,033)
Fair Value	828,702	395,518
Cash Equivalents	75,860	27,466
Marketable Securities	737,020	353,289
Restricted Cash	15,822	14,763
Level 1 \| Money market accounts
Debt Securities, Available-for-sale [Line Items]
Fair Value	26,298	32,057
Cash Equivalents	10,476	17,294
Marketable Securities	0	0
Restricted Cash	15,822	14,763
Level 2 \| U.S. government treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	323,212	131,643
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(891)	(651)
Fair Value	322,321	130,992
Cash Equivalents	62,392	10,172
Marketable Securities	259,929	120,820
Restricted Cash	0	0
Level 2 \| U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,362	47,229
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(355)	(333)
Fair Value	73,007	46,896
Cash Equivalents	2,992	0
Marketable Securities	70,015	46,896
Restricted Cash	0	0
Level 2 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	377,468	186,506
Gross Unrealized Gains	31	116
Gross Unrealized Losses	(1,830)	(1,049)
Fair Value	375,669	185,573
Cash Equivalents	0	0
Marketable Securities	375,669	185,573
Restricted Cash	0	$ 0
Level 2 \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,657
Gross Unrealized Gains	1
Gross Unrealized Losses	(14)
Fair Value	6,644
Cash Equivalents	0
Marketable Securities	6,644
Restricted Cash	0
Level 2 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,804
Gross Unrealized Gains	0
Gross Unrealized Losses	(41)
Fair Value	24,763
Cash Equivalents	0
Marketable Securities	24,763
Restricted Cash	$ 0

Debt Securities, Available-for-sale, Cash Equivalents And Restricted Cash

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Debt Securities, Available-for-sale, Cash Equivalents And Restricted Cash, Accumulated Gross Unrealized Gain, Before Tax

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Debt Securities, Available-for-sale, Cash Equivalents And Restricted Cash, Accumulated Gross Unrealized Loss, Before Tax

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Debt Securities, Available-for-sale, Cash Equivalents And Restricted Cash, Amortized Cost

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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), classified as current.

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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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Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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