Represents annual rental payments due under the sublease during the reporting period.

No definition available.

Name:	mnta_AnnualRentalPaymentsSublease
Namespace Prefix:	mnta_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Tenant Improvement Allowance

+ References

No definition available.

+ Details

Name:	mnta_TenantImprovementAllowance
Namespace Prefix:	mnta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- 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 http://asc.fasb.org/extlink&oid=77888415&loc=SL77918607-209975

+ Details

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

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717

+ Details

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

- Details

Name:	us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis=mnta_WestKendallSubleaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis=mnta_Bent320SubleaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis=mnta_A301BinneyStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies - Operating Leases (Details) $ in Millions		1 Months Ended	12 Months Ended
	Jul. 01, 2017 USD ($)	Sep. 30, 2016 USD ($) ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 28, 2013 ft²
Operating Leased Assets [Line Items]
Operating leases, rent expense			$ 26.8	$ 19.3	$ 18.5
West Kendall Sublease
Operating Leased Assets [Line Items]
Approximate Square Footage \| ft²			78,454
320 Bent Sublease
Operating Leased Assets [Line Items]
Approximate Square Footage \| ft²						104,678
Annual rental payments sublease			$ 8.1
301 Binney Street
Operating Leased Assets [Line Items]
Approximate Square Footage \| ft²		80,000				52,252
Termination of Lease	$ 1.1
Annual rental payments sublease		$ 6.1
Tenant improvement allowance		$ 9.9