Tax Incentive Agreement (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Tax Incentive Agreement
Minimum number of years for which jobs must be maintained	5 years
Job creation tax award		$ 1.1
Portion of job creation tax award recognized		0.20

Represents the amount of job creation tax award expected to be received under a Tax Incentive Agreement with the Massachusetts Life Sciences Center.

No definition available.

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