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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2025
Stock-Based Compensation Expense
Stock-based compensation expense recognized	$ 9,787	$ 7,309	$ 35,417	$ 13,099
Unrecognized stock-based compensation expense for all stock awards	$ 66,000		66,000
Period over which unamortized stock-based compensation expense to be recognized	2 years 8 months 12 days
Non-employee
Stock-Based Compensation Expense
Stock-based compensation expense recognized	$ 300	100	500	200
Stock options
Stock-Based Compensation Expense
Stock-based compensation expense recognized	7,979	7,291	28,771	13,045
RSUs
Stock-Based Compensation Expense
Stock-based compensation expense recognized	1,542		5,939
ESPP
Stock-Based Compensation Expense
Stock-based compensation expense recognized	266		701
Restricted stock awards
Stock-Based Compensation Expense
Stock-based compensation expense recognized		18	6	54
Restricted stock awards | Non-employee | Maximum
Stock-Based Compensation Expense
Stock-based compensation expense recognized	100	100	100	100
ACELYRIN
Stock-Based Compensation Expense
Additional stock-based compensation	2,100		12,900
ACELYRIN | Severance Plan
Stock-Based Compensation Expense
Additional stock-based compensation					$ 13,100
Research and development
Stock-Based Compensation Expense
Stock-based compensation expense recognized	5,773	3,720	16,717	6,805
Research and development | ACELYRIN | Severance Plan
Stock-Based Compensation Expense
Additional stock-based compensation	1,100		4,100
General and administrative
Stock-Based Compensation Expense
Stock-based compensation expense recognized	4,014	$ 3,589	18,700	$ 6,294
General and administrative | ACELYRIN | Severance Plan
Stock-Based Compensation Expense
Additional stock-based compensation	$ 1,000		$ 8,800

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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