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	Jan. 31, 2020	Jan. 31, 2019
Assets:
Cash equivalents, fair value	$ 436,580	$ 250,834
Short term investments, fair value	882,976	265,374
Total cash equivalents and short-term investments	1,319,556	516,208
Level 1
Assets:
Cash equivalents, fair value	416,584	247,426
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	416,584	247,426
Level 2
Assets:
Cash equivalents, fair value	19,996	3,408
Short term investments, fair value	882,976	265,374
Total cash equivalents and short-term investments	902,972	268,782
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
U.S. treasury securities
Assets:
Short term investments, fair value	576,598	195,897
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	576,598	195,897
U.S. treasury securities \| Level 3
Assets:
Short term investments, fair value	0	0
Corporate debt securities
Assets:
Short term investments, fair value	306,378	69,477
Corporate debt securities \| Level 1
Assets:
Short term investments, fair value	0	0
Corporate debt securities \| Level 2
Assets:
Short term investments, fair value	306,378	69,477
Corporate debt securities \| Level 3
Assets:
Short term investments, fair value	0	0
Money market funds
Assets:
Cash equivalents, fair value	416,584	247,426
Money market funds \| Level 1
Assets:
Cash equivalents, fair value	416,584	247,426
Money market funds \| Level 2
Assets:
Cash equivalents, fair value	0	0
Money market funds \| Level 3
Assets:
Cash equivalents, fair value	0	0
U.S. treasury securities
Assets:
Cash equivalents, fair value	19,996
U.S. treasury securities \| Level 1
Assets:
Cash equivalents, fair value	0
U.S. treasury securities \| Level 2
Assets:
Cash equivalents, fair value	19,996
U.S. treasury securities \| Level 3
Assets:
Cash equivalents, fair value	$ 0
Corporate debt securities
Assets:
Cash equivalents, fair value		3,408
Corporate debt securities \| Level 1
Assets:
Cash equivalents, fair value		0
Corporate debt securities \| Level 2
Assets:
Cash equivalents, fair value		3,408
Corporate debt securities \| 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=117815213&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)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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