Financial Instruments - Cash Equivalents, Marketable Securities and Restricted Cash (Details) - USD ($) $ in Thousands	Jul. 31, 2019	Jan. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 909,962	$ 793,932
Amortized Cost	908,933
Gross Unrealized Gains	4,790	900
Gross Unrealized Losses	(202)	(1,238)
Fair Value	951,209	836,632
Fair Value	913,521
Cash Equivalents	22,263	71,327
Marketable Securities	913,521	749,482
Restricted Cash	15,425	15,823
Level 1 \| Money market accounts
Debt Securities, Available-for-sale [Line Items]
Fair Value	36,659	43,038
Cash Equivalents	21,234	27,215
Marketable Securities	0	0
Restricted Cash	15,425	15,823
Level 2 \| U.S. government treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	348,463	315,329
Gross Unrealized Gains	1,454	208
Gross Unrealized Losses	(58)	(315)
Fair Value	349,859	315,222
Cash Equivalents	0	34,129
Marketable Securities	349,859	281,093
Restricted Cash	0	0
Level 2 \| U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	66,153	69,114
Gross Unrealized Gains	222	17
Gross Unrealized Losses	(22)	(154)
Fair Value	66,353	68,977
Cash Equivalents	0	9,983
Marketable Securities	66,353	58,994
Restricted Cash	0	0
Level 2 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	427,362	363,860
Gross Unrealized Gains	2,685	534
Gross Unrealized Losses	(111)	(757)
Fair Value	429,936	363,637
Cash Equivalents	1,029	0
Marketable Securities	428,907	363,637
Restricted Cash	0	0
Level 2 \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,480
Amortized Cost		7,965
Gross Unrealized Gains	100
Gross Unrealized Gains		36
Gross Unrealized Losses	0
Gross Unrealized Losses		0
Fair Value	8,580
Fair Value		8,001
Cash Equivalents	0	0
Marketable Securities	8,580	8,001
Restricted Cash	0	0
Level 2 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	59,504
Amortized Cost		37,664
Gross Unrealized Gains	329
Gross Unrealized Gains		105
Gross Unrealized Losses	(11)
Gross Unrealized Losses		(12)
Fair Value	59,822
Fair Value		37,757
Cash Equivalents	0	0
Marketable Securities	59,822	37,757
Restricted Cash	$ 0	$ 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, 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), 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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