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Restricted Cash (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Restricted cash    
Restricted cash $ 20,719us-gaap_RestrictedCashAndCashEquivalentsNoncurrent $ 20,719us-gaap_RestrictedCashAndCashEquivalentsNoncurrent
Security Bond Collateral    
Restricted cash    
Restricted cash 17,500us-gaap_RestrictedCashAndCashEquivalentsNoncurrent
/ us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis
= mnta_SecurityBondCollateralMember		  
Lease Collateral | West Kendall Sublease    
Restricted cash    
Restricted cash 2,500us-gaap_RestrictedCashAndCashEquivalentsNoncurrent
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= mnta_WestKendallSubleaseMember
/ us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis
= mnta_LeaseCollateralMember		  
Lease Collateral | 320 Bent Sublease    
Restricted cash    
Restricted cash $ 700us-gaap_RestrictedCashAndCashEquivalentsNoncurrent
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= mnta_Bent320SubleaseMember
/ us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis
= mnta_LeaseCollateralMember