Leases.

No definition available.

Name:	mgta_LeasesLineItems
Namespace Prefix:	mgta_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of payment made to the landlord for early termination of an operating lease.

+ References

No definition available.

+ Details

Name:	mgta_OperatingLeaseTerminationPayment
Namespace Prefix:	mgta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The size of the area being leased as part of an operating lease.

+ References

No definition available.

+ Details

Name:	mgta_OperatingLeasesGrossFloorAreaLeased
Namespace Prefix:	mgta_
Data Type:	dtr-types:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

A general description of the nature of the existing leasing arrangements of a lessee for all operating leases including, but not limited to: (1) rental escalation clauses; (2) renewal or purchase options; (3) guarantees or indemnities, if any, (4) restrictions imposed by lease arrangements; (5) unusual provisions or conditions; (6) contingent rentals, if any; and (7) lease expiration dates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 1)
-URI https://asc.fasb.org/extlink&oid=123403562&loc=d3e38371-112697

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 50
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=123389372&loc=d3e36991-112694

+ Details

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

- Definition

Amount of gain (loss) on termination of lease before expiration of lease term.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 40
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=123386189&loc=SL77918607-209975

+ Details

Name:	us-gaap_GainLossOnTerminationOfLease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in obligation for operating lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (g)(1)
-URI https://asc.fasb.org/extlink&oid=128292326&loc=SL77918686-209980

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInOperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of revenue recognized for the period under subleasing arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 55
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=123386226&loc=d3e41620-112719

+ Details

Name:	us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=126999549&loc=SL98516268-108586

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(1))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Amount of cash restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=126999549&loc=SL98516268-108586

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 954
-SubTopic 210
-Section 45
-Paragraph 5
-URI https://asc.fasb.org/extlink&oid=120413173&loc=SL116631458-115580

+ Details

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

- Details

Name:	us-gaap_TypeOfArrangementAxis=mgta_SubSubleaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseContractualTermAxis=mgta_OfficeAndLabSubleaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Leases - Additional Information (Detail) $ in Thousands	3 Months Ended
Leases - Additional Information (Detail) $ in Thousands	Mar. 31, 2023 USD ($) ft²	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)
Leases [Line Items]
Restricted Cash, Noncurrent	$ 0		$ 1,780
Restricted Cash, Current	$ 1,780		0
Property sublease, description	The Company had a sublease, as amended, for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease was subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and was set to expire in February 2028
Decrease in operating lease liability	$ (15,639)	$ (692)
Gain Loss On Termination Of Lease	$ (8,059)	0
Sub Sublease [Member]
Leases [Line Items]
Square feet of property subject to sublease \| ft²	26,114
Other income from sub-sublease	$ 800	$ 800
Office and Lab Sublease [Member]
Leases [Line Items]
Operating lease termination payment	14,800
Office and Lab Sublease [Member] \| Letter of Credit [Member]
Leases [Line Items]
Restricted Cash, Noncurrent			$ 1,800
Restricted Cash, Current	$ 1,800