Cash Equivalents, Marketable Securities and Restricted Cash (Details) - USD ($) $ in Thousands	Apr. 30, 2019	Jan. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 917,373	$ 793,932
Amortized Cost	877,157
Gross Unrealized Gains	2,316	900
Gross Unrealized Losses	(515)	(1,238)
Fair Value	943,748	836,632
Fair Value	878,958
Cash Equivalents	48,504	71,327
Marketable Securities	878,958	749,482
Restricted Cash	16,286	15,823
Level 1 \| Money market accounts
Debt Securities, Available-for-sale [Line Items]
Fair Value	24,574	43,038
Cash Equivalents	8,288	27,215
Marketable Securities	0	0
Restricted Cash	16,286	15,823
Level 2 \| U.S. government treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	382,113	315,329
Gross Unrealized Gains	534	208
Gross Unrealized Losses	(125)	(315)
Fair Value	382,522	315,222
Cash Equivalents	36,953	34,129
Marketable Securities	345,569	281,093
Restricted Cash	0	0
Level 2 \| U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	79,749	69,114
Gross Unrealized Gains	71	17
Gross Unrealized Losses	(79)	(154)
Fair Value	79,741	68,977
Cash Equivalents	0	9,983
Marketable Securities	79,741	58,994
Restricted Cash	0	0
Level 2 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	403,878	363,860
Gross Unrealized Gains	1,418	534
Gross Unrealized Losses	(311)	(757)
Fair Value	404,985	363,637
Cash Equivalents	3,263	0
Marketable Securities	401,722	363,637
Restricted Cash	0	0
Level 2 \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,472
Amortized Cost		7,965
Gross Unrealized Gains	66
Gross Unrealized Gains		36
Gross Unrealized Losses	0
Gross Unrealized Losses		0
Fair Value	8,538
Fair Value		8,001
Cash Equivalents	0	0
Marketable Securities	8,538	8,001
Restricted Cash	0	0
Level 2 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	43,161
Amortized Cost		37,664
Gross Unrealized Gains	227
Gross Unrealized Gains		105
Gross Unrealized Losses	0
Gross Unrealized Losses		(12)
Fair Value	43,388
Fair Value		37,757
Cash Equivalents	0	0
Marketable Securities	43,388	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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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 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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