Collaboration and License Agreements - Baxalta Collaboration Agreement (Details) - Baxalta Collaboration Agreement - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
One time cash payment	$ 51.2
Research and development revenue		$ 22.0

Aggregate revenue earned by the Company during the period under its collaboration agreements for reimbursement of research and development services, reimbursement of development costs, amortization of up-front payments and certain milestone payments.

No definition available.

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