Commitments and Contingencies - Legal Contingencies (Details) $ in Millions		1 Months Ended	12 Months Ended
Sep. 10, 2014 patent	Oct. 31, 2011 USD ($)	Dec. 31, 2016 USD ($)
Legal Contingencies
Orange Book-listed patents for 40 mg/ml Copaxone \| patent	2
Amount of security bond required to be posted by company and Sandoz in connection with the litigation		$ 100
Maximum amount of security bond for which the entity shall be responsible in the event of loss of case			$ 35

Represents the maximum amount of security bond for which the entity shall be responsible in the event of loss of case and the plaintiffs are able to prove that they suffered damages during the period preliminary injunction was in effect.

No definition available.

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