The Company and Basis of Presentation (Details) $ / shares in Units, $ in Thousands									3 Months Ended			9 Months Ended	12 Months Ended	127 Months Ended
	Oct. 15, 2025 USD ($) $ / shares shares	Nov. 29, 2024 shares	May 14, 2024 shares	Apr. 23, 2024 $ / shares shares	Mar. 20, 2024 USD ($) $ / shares shares	Mar. 18, 2024 shares	Dec. 11, 2023 shares	Jun. 22, 2023	Sep. 30, 2025 USD ($) $ / shares shares	Jun. 30, 2024 shares	Mar. 31, 2024 shares	Sep. 30, 2025 USD ($) segment $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares	Sep. 30, 2025 USD ($) $ / shares shares	Jun. 30, 2025 shares	Mar. 31, 2025 shares	Feb. 28, 2025 USD ($)	Sep. 30, 2024 shares	Sep. 06, 2024 USD ($)	Apr. 25, 2024 shares	Apr. 24, 2024 shares	Dec. 31, 2023 shares
Company and Basis of Presentation [Line Items]
Number of operating segments | segment												1
Proceeds from raising capital | $														$ 1,300,000
Accumulated deficit | $									$ 1,065,105			$ 1,065,105	$ 972,432	1,065,105
Marketable securities | $									$ 486,200			$ 486,200		$ 486,200
Common Stock
Company and Basis of Presentation [Line Items]
Exchange of Series A non-voting convertible preferred stock into common stock (in shares)										3,640,000
Shares converted (in shares)										10,198,000
Conversion Of Series B Non-Voting Convertible Preferred Stock | Common Stock
Company and Basis of Presentation [Line Items]
Shares converted (in shares)			10,198,320									10,198,320
At-The-Market Offering
Company and Basis of Presentation [Line Items]
Sale of stock, authorized amount | $																			$ 500,000
Common Stock and Series B Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Series B non-voting convertible preferred stock, par value (in dollars per share) | $ / shares					$ 0.0001
Conversion basis					40
Series B Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Conversion basis					40
Issuance of non-voting convertible preferred stock (in shares)					121,625	271,625	150,000
Preferred stock, par value (in dollars per share) | $ / shares									$ 0.0001			$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, outstanding (in shares)									16,667			16,667	16,667	16,667
Preferred stock, not automatically converted, outstanding (in shares)									16,667			16,667		16,667
Series B Non Voting Convertible Preferred Stock | Preferred Stock
Company and Basis of Presentation [Line Items]
Issuance of non-voting convertible preferred stock (in shares)											122,000
Preferred stock, outstanding (in shares)									17,000	17,000	0	17,000	17,000	17,000	17,000	17,000		17,000				0
Shares converted (in shares)										(255,000)
Series B Non Voting Convertible Preferred Stock | Conversion Of Series B Non-Voting Convertible Preferred Stock | Preferred Stock
Company and Basis of Presentation [Line Items]
Shares converted (in shares)			254,958									254,958
Series B Non Voting Convertible Preferred Stock | Private Placement
Company and Basis of Presentation [Line Items]
Sale of stock, price per share (in dollars per share) | $ / shares					$ 1,480
Sale of stock, proceeds received | $					$ 168,900
Placement agent and other offering costs | $					$ 11,200
Series A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares									$ 0.0001			$ 0.0001	$ 0.0001	$ 0.0001
Exchange of Series A non-voting convertible preferred stock into common stock (in shares)				90,992
Preferred stock, outstanding (in shares)									346,045			346,045	346,045	346,045						346,045	346,045
Series A Non Voting Convertible Preferred Stock | Preferred Stock
Company and Basis of Presentation [Line Items]
Exchange of Series A non-voting convertible preferred stock into common stock (in shares)										(91,000)
Preferred stock, outstanding (in shares)									346,000	346,000	437,000	346,000	346,000	346,000	346,000	346,000		346,000				437,000
Series A Non Voting Convertible Preferred Stock | Conversion Of Series A Non-Voting Convertible Preferred Stock | Preferred Stock
Company and Basis of Presentation [Line Items]
Shares converted (in shares)				90,992
Common Stock
Company and Basis of Presentation [Line Items]
Exchange of Series A non-voting convertible preferred stock into common stock (in shares)				3,639,680
Common Stock | At-The-Market Offering
Company and Basis of Presentation [Line Items]
Sale of stock, proceeds received | $													$ 20,500
Sale of stock, authorized amount | $																	$ 500,000
Sale of stock, aggregate offering price | $																			200,000
Number of shares of common stock sold (in shares)									0				777,432
Sale of stock, remaining authorized amount | $									$ 179,100			$ 179,100		$ 179,100
Common Stock | At-The-Market Offering | Maximum
Company and Basis of Presentation [Line Items]
Sale of stock, aggregate offering price | $																	$ 179,100		$ 179,100
Common Stock | October 2025 Underwritten Offering | Subsequent Events
Company and Basis of Presentation [Line Items]
Sale of stock, price per share (in dollars per share) | $ / shares	$ 18.50
Sale of stock, proceeds received | $	$ 296,500
Placement agent and other offering costs | $	$ 19,800
Number of shares of common stock sold (in shares)	17,094,594
Common Stock | Over-Allotment Option
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares)		1,091,250
Common Stock | Over-Allotment Option | Subsequent Events
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares)	2,229,729
Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Asset acquisition, stockholder payment period (in years)								3 years
Asset acquisition, cash payment, threshold period (in years)								1 year
Asset Acquisition | Series A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares				$ 0.0001