Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 22, 2023	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2016	Jun. 30, 2025	Jan. 01, 2025	Dec. 31, 2024	Jan. 01, 2024
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 13,100	$ 27,803	$ 35,617
2015 and 2016 and 2018 Equity Inducement Plan | Service Based Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based award, vesting period (in years)				4 years
Expiration term (in years)				10 years
2016 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual percentage of additional shares						5.00%
Additional number of shares available for issuance (in shares)								3,814,905		3,023,650
Common stock available for future issuance (in shares)		10,880,124		10,880,124
Outstanding option awards (in shares)		4,896,574		4,896,574
2018 Equity Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for future issuance (in shares)		7,556,574		7,556,574
Outstanding option awards (in shares)		6,637,231		6,637,231
2018 Equity Inducement Plan, Amended
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for future issuance, additional amount authorized (in shares)							750,000
Spyre 2023 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding option awards (in shares)							0
Spyre 2023 Equity Incentive Plan | Asset Acquisition
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares)	2,734
Parapyre Option Obligation
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 6,100		$ 13,000
2024 Parapyre Option Obligation | Outstanding Parapyre warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase shares (in shares)		1,532,591		1,532,591					848,184
Exercise price per warrant (in dollars per share)		$ 22.49		$ 22.49					$ 23.28
Number of warrants exercised (in shares)		0		0
2016 Employee Stock Purchase Plan | 2016 ESPP | Employee Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares sold (in shares)		17,298	14,053	34,175	16,383
Sale of stock, proceeds received		$ 200	$ 0	$ 600	$ 300