Subsequent Event (Details) - Subsequent Event - CSL License Agreement $ in Millions	Jan. 05, 2017 USD ($)
Subsequent Event
Non-refundable upfront payment	$ 50
Milestone payment on collaboration agreement	$ 550
Percentage of cost sharing arrangement under collaborative and license agreements	50.00%
Maximum
Subsequent Event
Percentage of contractual profits earned under collaborative arrangement	50.00%
Minimum
Subsequent Event
Percentage of contractual profits earned under collaborative arrangement	30.00%

Represents the percentage of cost sharing arrangement under collaborative and license agreements.

No definition available.

Name:	mnta_CollaborativeArrangementPercentageOfCostSharing
Namespace Prefix:	mnta_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of net profit shared on sales of the product if no third-party competitors are marketing an interchangeable generic version of Product and Equivalent Product.

+ References

No definition available.

+ Details

Name:	mnta_CollaborativeArrangementPercentageOfProfitSharing
Namespace Prefix:	mnta_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Non-refundable upfront payment received under the term of Collaboration Agreement.

+ References

No definition available.

+ Details

Name:	mnta_NonRefundableUpfrontPaymentReceived
Namespace Prefix:	mnta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of consideration recognized during the period for the milestone or milestones.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 28
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=68058744&loc=SL6892177-166501

+ Details

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

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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