Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479983/718-10-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479830/718-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_GranteeStatusAxis=us-gaap_ShareBasedPaymentArrangementNonemployeeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=alms_PerformanceOptionPlan2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=alms_AcelyrinIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=us-gaap_EmployeeSeveranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementOfIncomeLocationBalanceAxis=us-gaap_GeneralAndAdministrativeExpense
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementOfIncomeLocationBalanceAxis=us-gaap_ResearchAndDevelopmentExpense
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Stock-Based Compensation Expense
Stock-based compensation expense recognized	$ 14,273	$ 18,635	$ 26,612	$ 25,630
Unrecognized stock-based compensation expense for all stock awards	152,300		$ 152,300
Period over which unamortized stock-based compensation expense to be recognized			2 years 10 months 24 days
Non-employee
Stock-Based Compensation Expense
Stock-based compensation expense recognized	400	100	$ 700	300
2024 POP
Stock-Based Compensation Expense
Stock-based compensation expense recognized	900	2,100	2,200	4,300
Stock options
Stock-Based Compensation Expense
Stock-based compensation expense recognized	11,681	13,989	22,144	20,792
RSUs
Stock-Based Compensation Expense
Stock-based compensation expense recognized	2,230	4,397	3,790	4,397
ESPP
Stock-Based Compensation Expense
Stock-based compensation expense recognized	362	249	678	435
Restricted stock awards
Stock-Based Compensation Expense
Stock-based compensation expense recognized				6
ACELYRIN \| Severance Plan
Stock-Based Compensation Expense
Stock-based compensation expense recognized		10,800	0	10,800	$ 13,100
Location, Statement of Income, Balance [Axis]: us-gaap:GeneralAndAdministrativeExpense
Stock-Based Compensation Expense
Stock-based compensation expense recognized	5,630	11,410	10,617	14,686
Location, Statement of Income, Balance [Axis]: us-gaap:GeneralAndAdministrativeExpense \| ACELYRIN \| Severance Plan
Stock-Based Compensation Expense
Stock-based compensation expense recognized		7,800		7,800	9,000
Location, Statement of Income, Balance [Axis]: us-gaap:ResearchAndDevelopmentExpense
Stock-Based Compensation Expense
Stock-based compensation expense recognized	$ 8,643	7,225	$ 15,995	10,944
Location, Statement of Income, Balance [Axis]: us-gaap:ResearchAndDevelopmentExpense \| ACELYRIN \| Severance Plan
Stock-Based Compensation Expense
Stock-based compensation expense recognized		$ 3,000		$ 3,000	$ 4,100