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Commitments and Contingent Liabilities - FronThera Contingent Consideration (Details) - FronThera - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2024	Mar. 31, 2021	Dec. 31, 2022	Sep. 30, 2025
FronThera Contingent Consideration
Milestone payment obligation				$ 0.0
Clinical milestones
FronThera Contingent Consideration
Contingent consideration liability		$ 120.0
Milestone payment made	$ 23.0	70.0	$ 37.0
Approval milestones | Maximum
FronThera Contingent Consideration
Milestone payment made		$ 50.0

X

- Definition

Amount of milestone obligation in connection with asset acquisition.

+ References

No definition available.

+ Details

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Namespace Prefix:	alms_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of milestone payment made in connection with asset acquisition.

+ References

No definition available.

+ Details

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Namespace Prefix:	alms_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of liability recognized from contingent consideration in asset acquisition.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 50
-Name Accounting Standards Codification
-Section 15
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480123/805-50-15-3

+ Details

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

X

- 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 805
-SubTopic 50
-Name Accounting Standards Codification
-Section 15
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480123/805-50-15-3

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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