Financial Instruments - Summary of Cash Equivalents, Marketable Securities and Restricted Cash by Significant Investment Categories (Details) - USD ($) $ in Thousands	Apr. 30, 2017	Jan. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 373,987	$ 376,774
Gross Unrealized Gains	324	268
Gross Unrealized Losses	(769)	(830)
Fair Value	386,274	388,946
Cash Equivalents	6,324	13,226
Marketable Securities	367,218	362,986
Restricted Cash	12,732	12,734
Money market accounts \| Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	12,732	12,734
Cash Equivalents	0	0
Marketable Securities	0	0
Restricted Cash	12,732	12,734
U.S. government treasury notes \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	144,028	148,298
Gross Unrealized Gains	26	22
Gross Unrealized Losses	(313)	(289)
Fair Value	143,741	148,031
Cash Equivalents	6,324	13,226
Marketable Securities	137,417	134,805
Restricted Cash	0	0
U.S. government agencies \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	41,576	40,398
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(147)	(159)
Fair Value	41,430	40,241
Cash Equivalents	0	0
Marketable Securities	41,430	40,241
Restricted Cash	0	0
Corporate debt securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	186,005	185,701
Gross Unrealized Gains	295	242
Gross Unrealized Losses	(307)	(379)
Fair Value	185,993	185,564
Cash Equivalents	0	0
Marketable Securities	185,993	185,564
Restricted Cash	0	0
Foreign government bonds \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,378	2,377
Gross Unrealized Gains	2	2
Gross Unrealized Losses	(2)	(3)
Fair Value	2,378	2,376
Cash Equivalents	0	0
Marketable Securities	2,378	2,376
Restricted Cash	$ 0	$ 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
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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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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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Cash and equivalents whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements.

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