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Stockholders' Equity (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Feb. 28, 2014
Preferred Stock    
Exercise price (in dollars per share)   $ 34.12us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Rights [Member]    
Preferred Stock    
Exercise price (in dollars per share) $ 80.0us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
/ us-gaap_ClassOfWarrantOrRightAxis
= us-gaap_RightsMember
 
Exercise price before adjustment (in dollars per share) $ 250.0amag_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRightsBeforeAdjustment
/ us-gaap_ClassOfWarrantOrRightAxis
= us-gaap_RightsMember
 
Percentage of beneficial ownership to acquire to make the Rights exercisable 4.99%amag_PercentageOfBeneficialOwnershipOfPersonOrGroupOfAffiliatedOrAssociatedToMakeRightsExercisable
/ us-gaap_ClassOfWarrantOrRightAxis
= us-gaap_RightsMember
 
Rights [Member] | Minimum [Member]    
Preferred Stock    
Percentage of ownership before the NOL Amendment 20.00%amag_PercentageOfOwnershipBeforeNolAmendment
/ us-gaap_ClassOfWarrantOrRightAxis
= us-gaap_RightsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember