Financial Instruments - Summary of Cash Equivalents, Marketable Securities and Restricted Cash by Significant Investment Categories (Details) - USD ($) $ in Thousands	Oct. 31, 2017	Jan. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 376,430	$ 376,774
Gross Unrealized Gains	215	268
Gross Unrealized Losses	(934)	(830)
Fair Value	390,474	388,946
Cash Equivalents	6,374	13,226
Marketable Securities	369,337	362,986
Restricted Cash	14,763	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	14,763	12,734
Cash Equivalents	0	0
Marketable Securities	0	0
Restricted Cash	14,763	12,734
U.S. government treasury notes \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	144,297	148,298
Gross Unrealized Gains	6	22
Gross Unrealized Losses	(419)	(289)
Fair Value	143,884	148,031
Cash Equivalents	6,374	13,226
Marketable Securities	137,510	134,805
Restricted Cash	0	0
U.S. government agencies \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	44,517	40,398
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(205)	(159)
Fair Value	44,312	40,241
Cash Equivalents	0	0
Marketable Securities	44,312	40,241
Restricted Cash	0	0
Corporate debt securities \| Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	187,616	185,701
Gross Unrealized Gains	209	242
Gross Unrealized Losses	(310)	(379)
Fair Value	187,515	185,564
Cash Equivalents	0	0
Marketable Securities	187,515	185,564
Restricted Cash	$ 0	0
Foreign government bonds \| Level 2
Schedule Of Available For Sale Securities [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.

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