Name:	mnta_ShareBasedCompensationArrangementByShareBasedPaymentAwardApprovedIncreasesInSharePoolInYear2014
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Data Type:	xbrli:sharesItemType
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Represents the number of stock options and restricted stock awards approved increases in share pool under prior plans in year 2015

No definition available.

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Data Type:	xbrli:sharesItemType
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Represents the number of stock options and awards forfeited from June 12, 2013 to December 31, 2013.

No definition available.

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Data Type:	xbrli:sharesItemType
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Represents the number of stock options and restricted stock awards forfeited or expired under prior plans.

No definition available.

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Data Type:	xbrli:sharesItemType
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Represents the number of stock options and awards granted from June 12, 2013 to December 31, 2013.

No definition available.

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Data Type:	xbrli:sharesItemType
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Represents the percentage threshold combined voting power of all classes of stock held by employees considered for grant.

No definition available.

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Data Type:	num:percentItemType
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Number of shares available for issuance for each share granted.

No definition available.

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Data Type:	xbrli:sharesItemType
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Maximum number of shares reserved for the shares subject to other than the stock options that were granted under the Prior Plans.

No definition available.

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Maximum number of shares reserved for the shares subject to the options that were granted under the Prior Plans.

No definition available.

Name:	mnta_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesReservedForIssuanceMaximumNumberOfSharesForSharesSubjectToOptionsGrantedUnderPriorPlans
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Data Type:	xbrli:sharesItemType
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Period Type:	instant

Represents the fixed number of shares reserved for issuance under the plan.

No definition available.

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Data Type:	xbrli:sharesItemType
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Period Type:	instant

Represents the fixed number of shares reserved for issuance under the plan before amendment and restatement.

No definition available.

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Number of shares reserved for each share subject to an award other than a stock option that was granted under the Prior Plans.

No definition available.

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Data Type:	xbrli:sharesItemType
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Period Type:	instant

Number of shares reserved for each share subject to the option that was granted under the Prior Plans.

No definition available.

Name:	mnta_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesReservedForIssuanceNumberOfSharesForEachShareSubjectToOptionGrantedUnderPriorPlans
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Data Type:	xbrli:sharesItemType
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Period Type:	instant

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 14
-Section F

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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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.

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
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Data Type:	xbrli:stringItemType
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Period Type:	duration

Number of additional shares authorized for issuance under an established share-based compensation plan.

No definition available.

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

Net number of share options (or share units) granted during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Data Type:	xbrli:sharesItemType
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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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Data Type:	xbrli:durationItemType
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Period Type:	duration

Purchase price of common stock expressed as a percentage of its fair value.

No definition available.

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

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_ManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_PlanNameAxis=mnta_StockIncentivePlan2004Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PlanNameAxis=mnta_IncentiveAwardPlan2013Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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

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

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