Financial Instruments - Summary of Cash Equivalents, Marketable Securities and Restricted Cash by Significant Investment Categories (Details) - USD ($) $ in Thousands	Jul. 31, 2018	Jan. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 798,545	$ 365,378
Gross Unrealized Gains	105	116
Gross Unrealized Losses	(2,931)	(2,033)
Fair Value	824,766	395,518
Cash Equivalents	72,783	27,466
Marketable Securities	736,205	353,289	[1]
Restricted Cash	15,778	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	29,047	32,057
Cash Equivalents	13,269	17,294
Marketable Securities	0	0
Restricted Cash	15,778	14,763
U.S. government treasury notes | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	326,328	131,643
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(839)	(651)
Fair Value	325,489	130,992
Cash Equivalents	58,320	10,172
Marketable Securities	267,169	120,820
Restricted Cash	0	0
U.S. government agencies | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	76,443	47,229
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(374)	(333)
Fair Value	76,069	46,896
Cash Equivalents	1,194	0
Marketable Securities	74,875	46,896
Restricted Cash	0	0
Corporate debt securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	383,340	186,506
Gross Unrealized Gains	104	116
Gross Unrealized Losses	(1,713)	(1,049)
Fair Value	381,731	185,573
Cash Equivalents	0	0
Marketable Securities	381,731	185,573
Restricted Cash	0	$ 0
Foreign government bonds | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	5,151
Gross Unrealized Gains	1
Gross Unrealized Losses	(3)
Fair Value	5,149
Cash Equivalents	0
Marketable Securities	5,149
Restricted Cash	0
Asset-backed securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,283
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Value	7,281
Cash Equivalents	0
Marketable Securities	7,281
Restricted Cash	$ 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.