Collaborative Arrangement, Commercial Milestones Acquired By Collaborator, Percent

No definition available.

Name:	alny_CollaborativeArrangementCommercialMilestonesAcquiredByCollaboratorPercent
Namespace Prefix:	alny_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Collaborative Arrangement, Consideration Received

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementConsiderationReceived
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Collaborative Arrangement, Fixed Payment, Number Of Quarterly Payments

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementFixedPaymentNumberOfQuarterlyPayments
Namespace Prefix:	alny_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Collaborative Arrangement, Fixed Payment, Term

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementFixedPaymentTerm
Namespace Prefix:	alny_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Collaborative Arrangement, Maximum Funding

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementMaximumFunding
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Collaborative Arrangement, Milestone Payable

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementMilestonePayable
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Collaborative Arrangement, Royalties Acquired By Collaborator, Percent

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementRoyaltiesAndCommercialMilestonesAcquiredByCollaboratorPercent
Namespace Prefix:	alny_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Collaborative Arrangement, Royalties Payable, Percent

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementRoyaltiesPayablePercent
Namespace Prefix:	alny_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Collaborative Arrangement, Royalties Payable, Term

+ References

No definition available.

+ Details

Name:	alny_CollaborativeArrangementRoyaltiesPayableTerm
Namespace Prefix:	alny_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Expected Royalty Interest Payments

+ References

No definition available.

+ Details

Name:	alny_ExpectedRoyaltyInterestPayments
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Liability Related To The Sale Of Future Royalties [Line Items]

+ References

No definition available.

+ Details

Name:	alny_LiabilityRelatedToTheSaleOfFutureRoyaltiesLineItems
Namespace Prefix:	alny_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Cardiometabolic Programs

+ References

No definition available.

+ Details

Name:	alny_NumberOfCardiometabolicPrograms
Namespace Prefix:	alny_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Royalty And Development Funding Liability, Current

+ References

No definition available.

+ Details

Name:	alny_RoyaltyAndDevelopmentFundingLiabilityCurrent
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Royalty And Development Funding Liability, Fair Value

+ References

No definition available.

+ Details

Name:	alny_RoyaltyAndDevelopmentFundingLiabilityFairValue
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Royalty And Development Funding Liability, Noncurrent

+ References

No definition available.

+ Details

Name:	alny_RoyaltyAndDevelopmentFundingLiabilityNoncurrent
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Royalty Liability, Closing Costs

+ References

No definition available.

+ Details

Name:	alny_RoyaltyLiabilityClosingCosts
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Royalty Liability, Fair Value

+ References

No definition available.

+ Details

Name:	alny_RoyaltyLiabilityFairValue
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Royalty Liability, Interest Rate

+ References

No definition available.

+ Details

Name:	alny_RoyaltyLiabilityInterestRate
Namespace Prefix:	alny_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Royalty Liability, Noncurrent

+ References

No definition available.

+ Details

Name:	alny_RoyaltyLiabilityNoncurrent
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(a)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480657/210-10-S50-9

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482949/835-30-55-8

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482900/835-30-50-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-6

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_ProductOrServiceAxis=alny_VutrisiranMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=alny_ZilebesrianMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=alny_BlackstoneLifeSciencesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=alny_VutrisiranAndZilebesiranMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=us-gaap_CollaborativeArrangementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=alny_HELIOSBPhase3ClinicalTrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=alny_ALNAGTPhase2ClinicalTrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=alny_ALNAGTPhase3ClinicalTrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=alny_DevelopmentMilestoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=alny_VutrisiranMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=alny_ZilebesrianMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=alny_BlackstoneGroupIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

LIABILITIES RELATED TO THE SALE OF FUTURE ROYALTIES AND DEVELOPMENT FUNDING - Additional Information (Detail) $ in Thousands		1 Months Ended
	Sep. 30, 2026 USD ($)	Sep. 30, 2026 USD ($)	Jun. 30, 2026 USD ($)	Sep. 30, 2025 USD ($)	Mar. 31, 2025 USD ($) payment	Sep. 30, 2023 USD ($) payment	Aug. 31, 2020 USD ($) program	Apr. 30, 2020 USD ($)	Jan. 01, 2030	Dec. 31, 2025 USD ($)
Liability Related To The Sale Of Future Royalties [Line Items]
Number of cardiometabolic programs \| program							2
Liabilities related to the sale of future royalties and development funding	$ 272,037	$ 272,037								$ 220,068
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 1,462,828	$ 1,462,828								$ 1,470,341
Vutrisiran
Liability Related To The Sale Of Future Royalties [Line Items]
Effective interest rate	47.00%	47.00%								46.00%
Liabilities related to the sale of future royalties and development funding	$ 76,000	$ 76,000								$ 94,500
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 94,400	$ 94,400								$ 99,700
Zilebesrian
Liability Related To The Sale Of Future Royalties [Line Items]
Effective interest rate	31.00%	31.00%								32.00%
Liabilities related to the sale of future royalties and development funding	$ 6,900	$ 6,900								$ 6,800
Liabilities related to the sale of future royalties and development funding, net of current portion	18,300	18,300								10,300
Blackstone Life Sciences \| Vutrisiran and ALN-AGT \| Net revenues from collaborations
Liability Related To The Sale Of Future Royalties [Line Items]
Maximum funding							$ 150,000
Blackstone Life Sciences \| HELIOS-B Phase 3 Clinical Trial \| Net revenues from collaborations
Liability Related To The Sale Of Future Royalties [Line Items]
Maximum funding		70,000
Blackstone Life Sciences \| ALN-AGT Phase 2 Clinical Trial
Liability Related To The Sale Of Future Royalties [Line Items]
Development milestone payable						$ 84,500
Blackstone Life Sciences \| ALN-AGT Phase 2 Clinical Trial \| Net revenues from collaborations
Liability Related To The Sale Of Future Royalties [Line Items]
Maximum funding		26,000
Number of quarterly payments \| payment						16
Fixed payment, term (in years)						4 years
Blackstone Life Sciences \| ALN-AGT Phase 3 Clinical Trial \| Net revenues from collaborations
Liability Related To The Sale Of Future Royalties [Line Items]
Maximum funding	24,000		$ 36,000	$ 36,000
Development milestone payable						$ 243,000
Number of quarterly payments \| payment						16
Fixed payment, term (in years)						4 years
Blackstone Life Sciences \| Vutrisiran
Liability Related To The Sale Of Future Royalties [Line Items]
Development milestone payable					$ 175,000
Blackstone Life Sciences \| Vutrisiran \| Net revenues from collaborations
Liability Related To The Sale Of Future Royalties [Line Items]
Royalties payable, percent							1.00%
Royalties payable, term (in years)							10 years
Number of quarterly payments \| payment					8
Fixed payment, term (in years)					2 years
Blackstone Life Sciences \| Development Milestone
Liability Related To The Sale Of Future Royalties [Line Items]
Development milestone payable							$ 175,000
Vutrisiran
Liability Related To The Sale Of Future Royalties [Line Items]
Royalty and development funding liability, fair value	479,300	479,300								541,300
Zilebesrian
Liability Related To The Sale Of Future Royalties [Line Items]
Royalty and development funding liability, fair value	120,100	120,100								116,200
Blackstone Group Inc.
Liability Related To The Sale Of Future Royalties [Line Items]
Liabilities related to the sale of future royalties and development funding	189,200	189,200								118,800
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 1,350,000	$ 1,350,000								$ 1,360,000
Blackstone Group Inc. \| Net revenues from collaborations
Liability Related To The Sale Of Future Royalties [Line Items]
Collaborative arrangement, royalties and commercial milestones acquired by collaborator, percent								50.00%
Commercial milestones acquired by collaborator, percent								75.00%
Expected royalty interest payments								$ 1,000,000
Consideration received								$ 1,000,000
Interest rate	15.00%	15.00%								10.00%
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 1,539,305	$ 1,539,305								$ 1,479,209
Closing costs	7,700	7,700								8,300
Fair value of liability	$ 2,030,000	$ 2,030,000								$ 1,610,000
Blackstone Group Inc. \| Net revenues from collaborations \| Forecast
Liability Related To The Sale Of Future Royalties [Line Items]
Collaborative arrangement, royalties and commercial milestones acquired by collaborator, percent									55.00%