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	Apr. 30, 2020	Jan. 31, 2020
Assets:
Cash equivalents, fair value	$ 503,214	$ 436,580
Short term investments, fair value	827,556	882,976
Total cash equivalents and short-term investments	1,330,770	1,319,556
U.S. treasury securities
Assets:
Cash equivalents, fair value		19,996
Short term investments, fair value	580,077	576,598
Corporate debt securities
Assets:
Short term investments, fair value	247,479	306,378
Money market funds
Assets:
Cash equivalents, fair value	497,208	416,584
Corporate debt securities
Assets:
Cash equivalents, fair value	6,006
Level 1
Assets:
Cash equivalents, fair value	497,208	416,584
Short term investments, fair value	0	0
Total cash equivalents and short-term investments	497,208	416,584
Level 1 \| U.S. treasury securities
Assets:
Cash equivalents, fair value		0
Short term investments, fair value	0	0
Level 1 \| Corporate debt securities
Assets:
Short term investments, fair value	0	0
Level 1 \| Money market funds
Assets:
Cash equivalents, fair value	497,208	416,584
Level 1 \| Corporate debt securities
Assets:
Cash equivalents, fair value	0
Level 2
Assets:
Cash equivalents, fair value	6,006	19,996
Short term investments, fair value	827,556	882,976
Total cash equivalents and short-term investments	833,562	902,972
Level 2 \| U.S. treasury securities
Assets:
Cash equivalents, fair value		19,996
Short term investments, fair value	580,077	576,598
Level 2 \| Corporate debt securities
Assets:
Short term investments, fair value	247,479	306,378
Level 2 \| Money market funds
Assets:
Cash equivalents, fair value	0	0
Level 2 \| Corporate debt securities
Assets:
Cash equivalents, fair value	6,006
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
Level 3 \| U.S. treasury securities
Assets:
Cash equivalents, fair value		0
Short term investments, fair value	0	0
Level 3 \| Corporate debt securities
Assets:
Short term investments, fair value	0	0
Level 3 \| Money market funds
Assets:
Cash equivalents, fair value	0	$ 0
Level 3 \| Corporate debt securities
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)
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