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Commitments and Contingencies - Additional Information (Detail) (USD $)
 1 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Commitments and Contingencies [Line Items]                        
Future purchase commitments   $ 104,500,000us-gaap_PurchaseObligation               $ 104,500,000us-gaap_PurchaseObligation    
Future purchase commitments expected to be incurred in 2015   59,300,000us-gaap_PurchaseObligationDueInNextTwelveMonths               59,300,000us-gaap_PurchaseObligationDueInNextTwelveMonths    
Future purchase commitments expected to be incurred past 2015   45,200,000alny_PurchaseObligationThereafter               45,200,000alny_PurchaseObligationThereafter    
Lease extension period 5 years                      
Lease extended expiration date Sep. 30, 2021                      
Operating lease obligation through 2021   18,955,000us-gaap_ContractualObligation               18,955,000us-gaap_ContractualObligation    
Lease expiration date                     2016-09  
Rent expense                   8,900,000us-gaap_OperatingLeasesRentExpenseNet 4,900,000us-gaap_OperatingLeasesRentExpenseNet 6,400,000us-gaap_OperatingLeasesRentExpenseNet
Operating expenses   69,731,000us-gaap_OperatingExpenses 56,171,000us-gaap_OperatingExpenses 52,300,000us-gaap_OperatingExpenses 277,339,000us-gaap_OperatingExpenses 40,439,000us-gaap_OperatingExpenses 41,225,000us-gaap_OperatingExpenses 29,999,000us-gaap_OperatingExpenses 28,446,000us-gaap_OperatingExpenses 455,541,000us-gaap_OperatingExpenses 140,109,000us-gaap_OperatingExpenses 196,181,000us-gaap_OperatingExpenses
One time settlement payment                       65,000,000alny_CrossLicenseRestructuringExpense
Tekmira Restructured Cross-License Agreement                        
Commitments and Contingencies [Line Items]                        
Operating expenses                       65,000,000us-gaap_OperatingExpenses
/ us-gaap_LitigationCaseAxis
= alny_TekmiraRestructuredCrossLicenseAgreementMember
Future milestone payments                     5,000,000alny_RestructuredCrossLicenseMilestonePayments
/ us-gaap_LitigationCaseAxis
= alny_TekmiraRestructuredCrossLicenseAgreementMember		  
Milestone paid           5,000,000alny_MilestonesPaid
/ us-gaap_LitigationCaseAxis
= alny_TekmiraRestructuredCrossLicenseAgreementMember		            
Tekmira Restructured Cross-License Manufacturing Contracts and Service Agreement                        
Commitments and Contingencies [Line Items]                        
One time settlement payment                       30,000,000alny_CrossLicenseRestructuringExpense
/ us-gaap_LitigationCaseAxis
= alny_TekmiraRestructuredCrossLicenseManufacturingContractsAndServiceAgreementMember
Tekmira Restructured Cross-License Buy-Down                        
Commitments and Contingencies [Line Items]                        
One time settlement payment                       35,000,000alny_CrossLicenseRestructuringExpense
/ us-gaap_LitigationCaseAxis
= alny_TekmiraRestructuredCrossLicenseBuyDownMember
Contingent milestone payments                       10,000,000alny_ContingentMilestonePaymentsAggregateAmount
/ us-gaap_LitigationCaseAxis
= alny_TekmiraRestructuredCrossLicenseBuyDownMember
Bmr Fresh Pond Research Park Llc                        
Commitments and Contingencies [Line Items]                        
Lease extension period                   5 years    
Area of office and laboratory space held under operating lease   15,000alny_NetRentableAreaUnderLease
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		               15,000alny_NetRentableAreaUnderLease
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		    
Lease expiration date                   Aug. 31, 2017    
Number 0f lease extension options                   2alny_NumberOfLeaseExtensionOptions
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		    
Lease incentive received   1,900,000us-gaap_IncentiveFromLessor
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		       200,000us-gaap_IncentiveFromLessor
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		       1,900,000us-gaap_IncentiveFromLessor
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		 200,000us-gaap_IncentiveFromLessor
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember		 1,800,000us-gaap_IncentiveFromLessor
/ alny_LeaseAgreementAxis
= alny_BmrFreshPondResearchParkLlcMember
Third Street                        
Commitments and Contingencies [Line Items]                        
Operating lease obligation through 2021 37,800,000us-gaap_ContractualObligation
/ alny_LeaseAgreementAxis
= alny_ThirdStreetMember		       $ 37,800,000us-gaap_ContractualObligation
/ alny_LeaseAgreementAxis
= alny_ThirdStreetMember		              
Area of office and laboratory space held under operating lease   129,000alny_NetRentableAreaUnderLease
/ alny_LeaseAgreementAxis
= alny_ThirdStreetMember		               129,000alny_NetRentableAreaUnderLease
/ alny_LeaseAgreementAxis
= alny_ThirdStreetMember		    
Operating Lease Amendment | Third Street | Optional                        
Commitments and Contingencies [Line Items]                        
Lease extension period 5 years