Common stock outstanding percentage to be declared as an increase in grants.

No definition available.

Name:	evlo_CommonStockOutstandingPercentageToBeDeclaredAsAnIncreaseInGrants
Namespace Prefix:	evlo_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Authorized Increase Decrease

+ References

No definition available.

+ Details

Name:	evlo_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedIncreaseDecrease
Namespace Prefix:	evlo_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share Based Payment Award, Number Of Shares Repurchased

+ References

No definition available.

+ Details

Name:	evlo_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesRepurchased
Namespace Prefix:	evlo_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Sharebased Compensation Arrangementby Sharebased Payment Award Optionsand Non Option Equity Instruments Exercised

+ References

No definition available.

+ Details

Name:	evlo_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsandNonOptionEquityInstrumentsExercised
Namespace Prefix:	evlo_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Sharebased Compensation Arrangementby Sharebased Payment Award Optionsand Non Option Equity Instruments Forfeitures

+ References

No definition available.

+ Details

Name:	evlo_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsandNonOptionEquityInstrumentsForfeitures
Namespace Prefix:	evlo_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Sharebased Compensation Arrangementby Sharebased Payment Award Optionsand Non Option Equity Instruments Granted

+ References

No definition available.

+ Details

Name:	evlo_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsandNonOptionEquityInstrumentsGranted
Namespace Prefix:	evlo_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost to be recognized for option under share-based payment arrangement.

+ References

+ Details

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

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Number of additional shares authorized for issuance under share-based payment arrangement.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum number of shares that may be issued in accordance with the plan as a proportion of outstanding capital stock.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares issued under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of share options (or share units) exercised during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=evlo_TwoThousandAndEighteenStockIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=evlo_NonEmployeeOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=evlo_TwoThousandAndFifteenStockIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-Based Compensation - Narrative (Details) - USD ($)					3 Months Ended
	Jan. 01, 2020	Jan. 01, 2019	May 08, 2018	Apr. 18, 2018	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares canceled (in shares)					560,730
Number of shares exercised (in shares)					137,213
Weighted average fair value of options granted (in dollars per share)					$ 4.81	$ 8.82
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense					$ 19,100,000
Compensation cost not yet recognized, period for recognition					2 years 10 months 13 days
2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized for issuance (in shares)				336,356
Number of shares canceled (in shares)					547,883
Number of shares exercised (in shares)					0
Number of common stock available for future grant (in shares)					1,552,557
Unrecognized compensation expense					$ 18,000.0
Percentage of outstanding shares				1.00%
2018 Stock Incentive Plan \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2018 Stock Incentive Plan \| Minimum \| Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
2018 Stock Incentive Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares originally authorized for issuance (in shares)			1,344,692
Common stock outstanding percentage to be declared as an increase in grants					4.00%
Number of additional shares authorized for issuance (in shares)	1,286,824	1,273,031
Options granted, maximum expiration period					10 years
Number of shares issued for purchase (in shares)					3,573,326
2018 Stock Incentive Plan \| Maximum \| Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares originally authorized for issuance (in shares)			0
Number of shares canceled (in shares)					673,453
Number of common stock available for future grant (in shares)					5,417,044
Number of shares, options and other equity awards granted (in shares)					5,758,518
Number of shares, options and other equity awards exercised (in shares)					1,322,481
Number of shares, options and other equity awards canceled (in shares)					1,109,462
Number of shares repurchased (in shares)					18,468
Number of shares no longer authorized (in shares)			113,006
2015 Stock Incentive Plan \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2015 Stock Incentive Plan \| Minimum \| Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
2015 Stock Incentive Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, maximum expiration period					10 years
2015 Stock Incentive Plan \| Maximum \| Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years