Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Oct. 31, 2018	Jan. 31, 2018
Assets:
Cash equivalents, fair value	$ 163,621	$ 90,770
Short term investments, fair value	350,105	101,765
Total cash equivalents and short-term investments	513,726	192,535
Level 1
Assets:
Cash equivalents, fair value	158,639	90,770
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	158,639	90,770
Level 2
Assets:
Cash equivalents, fair value	4,982	0
Short term investments, fair value	350,105	101,765
Total cash equivalents and short-term investments	355,087	101,765
Level 3
Assets:
Cash equivalents, fair value	0	0
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	0	0
Commercial paper
Assets:
Short term investments, fair value	99,039	15,946
Commercial paper \| Level 1
Assets:
Short term investments, fair value	0	0
Commercial paper \| Level 2
Assets:
Short term investments, fair value	99,039	15,946
Commercial paper \| Level 3
Assets:
Short term investments, fair value	0	0
U.S. treasury securities
Assets:
Short term investments, fair value	211,156	61,738
U.S. treasury securities \| Level 1
Assets:
Short term investments, fair value	0	0
U.S. treasury securities \| Level 2
Assets:
Short term investments, fair value	211,156	61,738
U.S. treasury securities \| Level 3
Assets:
Short term investments, fair value	0	0
Corporate debt securities
Assets:
Short term investments, fair value	39,910	24,081
Corporate debt securities \| Level 1
Assets:
Short term investments, fair value	0	0
Corporate debt securities \| Level 2
Assets:
Short term investments, fair value	39,910	24,081
Corporate debt securities \| Level 3
Assets:
Short term investments, fair value	0	0
Money market funds
Assets:
Cash equivalents, fair value	158,639	90,770
Money market funds \| Level 1
Assets:
Cash equivalents, fair value	158,639	90,770
Money market funds \| Level 2
Assets:
Cash equivalents, fair value	0	0
Money market funds \| Level 3
Assets:
Cash equivalents, fair value	0	$ 0
Commercial paper
Assets:
Cash equivalents, fair value	4,982
Commercial paper \| Level 1
Assets:
Cash equivalents, fair value	0
Commercial paper \| Level 2
Assets:
Cash equivalents, fair value	4,982
Commercial paper \| Level 3
Assets:
Cash equivalents, fair value	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes 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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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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