Financial Instruments - Fair Value of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jan. 31, 2018	Jan. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 365,378	$ 376,774
Gross Unrealized Gains	116	268
Gross Unrealized Losses	(2,033)	(830)
Fair Value	395,518	388,946
Cash Equivalents	27,466	13,226
Marketable Securities	353,289	362,986
Restricted Cash	14,763	12,734
Money market accounts \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	32,057	12,734
Cash Equivalents	17,294	0
Marketable Securities	0	0
Restricted Cash	14,763	12,734
U.S. government treasury notes \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	131,643	148,298
Gross Unrealized Gains	0	22
Gross Unrealized Losses	(651)	(289)
Fair Value	130,992	148,031
Cash Equivalents	10,172	13,226
Marketable Securities	120,820	134,805
Restricted Cash	0	0
U.S. government agencies \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	47,229	40,398
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(333)	(159)
Fair Value	46,896	40,241
Cash Equivalents	0	0
Marketable Securities	46,896	40,241
Restricted Cash	0	0
Corporate debt securities \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	186,506	185,701
Gross Unrealized Gains	116	242
Gross Unrealized Losses	(1,049)	(379)
Fair Value	185,573	185,564
Cash Equivalents	0	0
Marketable Securities	185,573	185,564
Restricted Cash	$ 0	0
Foreign government bonds \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		2,377
Gross Unrealized Gains		2
Gross Unrealized Losses		(3)
Fair Value		2,376
Cash Equivalents		0
Marketable Securities		2,376
Restricted Cash		$ 0

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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