Financial Instruments - Summary of Cash Equivalents, Marketable Securities and Restricted Cash by Significant Investment Categories (Details) - USD ($) $ in Thousands	Apr. 30, 2018	Jan. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 375,869	$ 365,378
Gross Unrealized Gains	77	116
Gross Unrealized Losses	(2,710)	(2,033)
Fair Value	397,983	395,518
Cash Equivalents	18,667	27,466
Marketable Securities	362,817	353,289	[1]
Restricted Cash	16,499	14,763
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	24,747	32,057
Cash Equivalents	8,248	17,294
Marketable Securities	0	0
Restricted Cash	16,499	14,763
U.S. government treasury notes | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	146,112	131,643
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(774)	(651)
Fair Value	145,339	130,992
Cash Equivalents	10,419	10,172
Marketable Securities	134,920	120,820
Restricted Cash	0	0
U.S. government agencies | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	40,674	47,229
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(390)	(333)
Fair Value	40,284	46,896
Cash Equivalents	0	0
Marketable Securities	40,284	46,896
Restricted Cash	0	0
Corporate debt securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	189,083	186,506
Gross Unrealized Gains	76	116
Gross Unrealized Losses	(1,546)	(1,049)
Fair Value	187,613	185,573
Cash Equivalents	0	0
Marketable Securities	187,613	185,573
Restricted Cash	$ 0	$ 0

[1] *Prior period information has been adjusted to reflect the adoption impact of Accounting Standards Codification 606, Revenue from Contracts with Customers (ASC 606), which we adopted on February 1, 2018.