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, 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/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 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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 3: 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 4: 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 5: 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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-10

+ Details

Name:	us-gaap_DerivativeLiabilitiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of equity (deficit) attributable to parent. Excludes temporary equity and equity attributable to noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 28
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-28

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(31))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 852
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481372/852-10-55-10

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-12

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(19))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.6-05(4))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-2

Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(4)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(7))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3

Reference 14: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 15: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 323
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481687/323-10-50-3

Reference 16: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 4.E)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-2

+ Details

Name:	us-gaap_StockholdersEquity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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_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:

- 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:

LIABILITIES RELATED TO THE SALE OF FUTURE ROYALTIES AND DEVELOPMENT FUNDING - Additional Information (Detail) $ in Thousands		1 Months Ended
	Dec. 31, 2025 USD ($)	Sep. 30, 2025 USD ($)	Mar. 31, 2025 payment	Aug. 31, 2020 USD ($) program payment	Apr. 30, 2020 USD ($)	Jan. 01, 2030	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Liability Related To The Sale Of Future Royalties [Line Items]
Number of cardiometabolic programs \| program				2
Stockholder's equity	$ 789,176						$ 67,088	$ (220,644)	$ (158,223)
Liabilities related to the sale of future royalties and development funding	220,068						113,018
Liabilities related to the sale of future royalties and development funding, net of current portion	1,470,341						1,334,353
Development derivative liability	0						93,780
Development derivative liability, net of current portion	$ 0						393,139
Cost of borrowing (as percent)	10.00%
Blackstone Life Sciences
Liability Related To The Sale Of Future Royalties [Line Items]
Cost of borrowing (as percent)	7.00%
Accumulated Deficit
Liability Related To The Sale Of Future Royalties [Line Items]
Stockholder's equity	$ (6,702,524)						(7,287,748)	(7,009,591)	$ (6,569,349)
Cumulative Effect, Period of Adoption, Adjustment
Liability Related To The Sale Of Future Royalties [Line Items]
Stockholder's equity							271,477
Cumulative Effect, Period of Adoption, Adjustment \| Accumulated Deficit
Liability Related To The Sale Of Future Royalties [Line Items]
Stockholder's equity							271,477
Vutrisiran
Liability Related To The Sale Of Future Royalties [Line Items]
Effective interest rate	46.00%
Liabilities related to the sale of future royalties and development funding	$ 94,500
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 99,700
Zilebesrian
Liability Related To The Sale Of Future Royalties [Line Items]
Effective interest rate	32.00%
Liabilities related to the sale of future royalties and development funding	$ 6,800
Liabilities related to the sale of future royalties and development funding, net of current portion	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	6,000	$ 18,000
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
Blackstone Group Inc.
Liability Related To The Sale Of Future Royalties [Line Items]
Liabilities related to the sale of future royalties and development funding	118,800						112,100
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 1,360,000						$ 1,340,000
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 (as percent)	10.00%						10.00%
Liabilities related to the sale of future royalties and development funding, net of current portion	$ 1,479,209						$ 1,447,371	$ 1,377,239
Closing costs	8,300						$ 9,100
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%
Vutrisiran
Liability Related To The Sale Of Future Royalties [Line Items]
Royalty and development funding liability, fair value	541,300
Zilebesrian
Liability Related To The Sale Of Future Royalties [Line Items]
Royalty and development funding liability, fair value	$ 116,200