Fair Value Measurements and Marketable Securities - Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Jul. 31, 2020	Jan. 31, 2020
Assets
Marketable securities	$ 0	$ 647,266
Recurring
Assets
Cash equivalents	0	245,319
Marketable securities	0	647,266
Total assets	0	892,585
Recurring \| Money market funds
Assets
Cash equivalents	0	205,379
Recurring \| Corporate debt securities
Assets
Cash equivalents	0	39,940
Marketable securities	0	495,022
Recurring \| U.S. treasury securities
Assets
Marketable securities	0	84,431
Recurring \| Asset backed securities
Assets
Marketable securities	0	67,813
Level 1 \| Recurring
Assets
Cash equivalents	0	205,379
Marketable securities	0	84,431
Total assets	0	289,810
Level 1 \| Recurring \| Money market funds
Assets
Cash equivalents	0	205,379
Level 1 \| Recurring \| Corporate debt securities
Assets
Cash equivalents	0	0
Marketable securities	0	0
Level 1 \| Recurring \| U.S. treasury securities
Assets
Marketable securities	0	84,431
Level 1 \| Recurring \| Asset backed securities
Assets
Marketable securities	0	0
Level 2 \| Recurring
Assets
Cash equivalents	0	39,940
Marketable securities	0	562,835
Total assets	0	602,775
Level 2 \| Recurring \| Money market funds
Assets
Cash equivalents	0	0
Level 2 \| Recurring \| Corporate debt securities
Assets
Cash equivalents	0	39,940
Marketable securities	0	495,022
Level 2 \| Recurring \| U.S. treasury securities
Assets
Marketable securities	0	0
Level 2 \| Recurring \| Asset backed securities
Assets
Marketable securities	0	67,813
Level 3 \| Recurring
Assets
Cash equivalents	0	0
Marketable securities	0	0
Total assets	0	0
Level 3 \| Recurring \| Money market funds
Assets
Cash equivalents	0	0
Level 3 \| Recurring \| Corporate debt securities
Assets
Cash equivalents	0	0
Marketable securities	0	0
Level 3 \| Recurring \| U.S. treasury securities
Assets
Marketable securities	0	0
Level 3 \| Recurring \| Asset backed securities
Assets
Marketable securities	$ 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

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26626-111562

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: