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Share-Based Payments (Details 2) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 0 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 29, 2015
Mar. 28, 2015
Share-Based Payments        
Allocated share-based compensation $ (4.4)us-gaap_AllocatedShareBasedCompensationExpense $ 3.5us-gaap_AllocatedShareBasedCompensationExpense    
Awards vesting on the one-year anniversary        
Share-Based Payments        
Award vesting period 1 year      
Additional disclosures        
Vesting percentage 25.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_VestingAxis
= mnta_AwardsVestingOnOneYearAnniversaryMember
     
Awards vesting on the quarterly basis        
Share-Based Payments        
Award vesting period 3 years      
Additional disclosures        
Vesting percentage 6.25%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_VestingAxis
= mnta_AwardsVestingOnQuarterlyBasisMember
     
Stock options        
Weighted average assumptions used in fair value calculations        
Expected volatility (as a percent) 61.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
67.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Expected life (years) 6 years 2 months 12 days 6 years 2 months 12 days    
Risk-free interest rate (as a percent) 1.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
2.20%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Additional disclosures        
Remaining unrecognized compensation cost related to nonvested awards 20.5us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Weighted average remaining requisite service period 2 years 9 months 18 days      
Employee Stock Purchase Plan        
Share-Based Payments        
Allocated share-based compensation 0.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
0.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Weighted average assumptions used in fair value calculations        
Expected volatility (as a percent) 61.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
66.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Expected life (years) 6 months 6 months    
Risk-free interest rate (as a percent) 0.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
0.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Additional disclosures        
Exercised (in shares) (225,400)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
     
Shares issued during period 58,398us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
     
Restricted stock awards        
Share-Based Payments        
Allocated share-based compensation (6.9)us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
1.2us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Additional disclosures        
Remaining unrecognized compensation cost related to nonvested awards 14.2us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Weighted average remaining requisite service period 1 year 8 months 12 days      
Non-vested shares of restricted stock, Number of Shares        
Nonvested at the beginning of the period (in shares) 1,174,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Granted (in shares) 250,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Vested (in shares) (85,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Forfeited (in shares) (127,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Nonvested at the end of the period (in shares) 1,212,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Non-vested shares of restricted stock, Weighted Average Grant Date Fair Value        
Nonvested at the beginning of the period (in dollars per share) $ 15.15us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Granted (in dollars per share) $ 13.02us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Vested (in dollars per share) $ 16.64us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Forfeited (in dollars per share) $ 14.57us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Nonvested at the end of the period (in dollars per share) $ 14.67us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Time-based restricted stock awards        
Non-vested shares of restricted stock, Number of Shares        
Nonvested at the end of the period (in shares) 544,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= mnta_TimeBasedRestrictedStockAwardMember
     
Time-based restricted stock awards | Officers        
Non-vested shares of restricted stock, Number of Shares        
Granted (in shares) 250,087us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= mnta_TimeBasedRestrictedStockAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_OfficerMember
     
Performance-based restricted stock awards        
Share-Based Payments        
Award vesting period     1 year 1 year
Allocated share-based compensation 3.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
  9.8us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
10.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
Reversal of cumulative compensation cost $ 10.5mnta_ReversalOfCumulativeCompensationCost
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
     
Non-vested shares of restricted stock, Number of Shares        
Forfeited (in shares)     (117,898)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
 
Nonvested at the end of the period (in shares) 668,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
     
Performance-based restricted stock awards | Shares vesting upon FDA approval        
Additional disclosures        
Vesting percentage     50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
/ us-gaap_VestingAxis
= mnta_AwardsVestingUponFoodAndDrugAdministrationApprovalMember
50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
/ us-gaap_VestingAxis
= mnta_AwardsVestingUponFoodAndDrugAdministrationApprovalMember
Performance-based restricted stock awards | Awards vesting on the one-year anniversary of approval from FDA        
Share-Based Payments        
Percentage of reduction on original number of shares     15.00%mnta_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationAndForfeitureRightsPercentage
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
/ us-gaap_VestingAxis
= mnta_AwardsVestingOnOneYearAnniversaryOfApprovalFromFoodAndDrugAdministrationApprovalMember
 
Additional disclosures        
Vesting percentage     50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
/ us-gaap_VestingAxis
= mnta_AwardsVestingOnOneYearAnniversaryOfApprovalFromFoodAndDrugAdministrationApprovalMember
50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedRestrictedStockAwardMember
/ us-gaap_VestingAxis
= mnta_AwardsVestingOnOneYearAnniversaryOfApprovalFromFoodAndDrugAdministrationApprovalMember
Performance-based restricted stock awards, M356        
Non-vested shares of restricted stock, Number of Shares        
Granted (in shares)       949,620us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= mnta_PerformanceBasedM356RestrictedStockAwardMember