Organization and Nature of the Business (Details) $ / shares in Units, $ in Thousands						3 Months Ended
	Feb. 06, 2025 USD ($) installment $ / shares	Jul. 17, 2024 USD ($) shares	Jul. 01, 2024 USD ($) $ / shares shares	Jun. 20, 2024	Mar. 29, 2024 $ / shares	Mar. 31, 2025 USD ($) item	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)	Mar. 13, 2024 item
Organization and Nature of the Business
Number of wholly owned subsidiaries | item						1			2
Reverse stock split ratio				0.2139
Stock option, exercise price ($ per share) | $ / shares					$ 8.84
Preferred stock conversion ratio			0.2139
Termination fee	$ 10,000
Threshold percentage of issued and outstanding shares held	50.00%
Merger transaction costs						$ 7,700
Net loss						98,963	$ 49,849
Cash used in operating activities						80,355	$ 44,262
Accumulated deficit						(757,514)		$ (658,551)
Cash, cash equivalents and marketable securities						$ 208,700
The Combined Company
Organization and Nature of the Business
Ownership percentage	52.00%
ACELYRIN
Organization and Nature of the Business
Stock option, exercise price ($ per share) | $ / shares	$ 18
Exchange ratio	0.4814
Termination fee	$ 10,000
ACELYRIN | Performance based RSU
Organization and Nature of the Business
Percentage of performance conditions achieved	100.00%
ACELYRIN | Merger closes before May 15, 2025 | Performance based RSU
Organization and Nature of the Business
Number of installments | installment	3
ACELYRIN | Merger closes after May 15, 2025 | Performance based RSU
Organization and Nature of the Business
Number of installments | installment	2
ACELYRIN | The Combined Company
Organization and Nature of the Business
Ownership percentage	48.00%
Class A common stock
Organization and Nature of the Business
Redeemable convertible preferred stock | shares			28,855,656
Class B common stock
Organization and Nature of the Business
Redeemable convertible preferred stock | shares			7,184,908
IPO
Organization and Nature of the Business
Number of shares issued and sold | shares			13,125,000
Issue price per share | $ / shares			$ 16
Net proceeds after deducting underwriting discounts and commissions			$ 193,300
Payments of underwriting discounts and commission			$ 16,700
Concurrent Private Placement
Organization and Nature of the Business
Percentage of ownership held by investor		5.00%
Additional number of shares purchased | shares		2,500,000
Aggregate gross and net proceeds		$ 40,000