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Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 22, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2016	Jan. 01, 2024	Jan. 01, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation		$ 13,100	$ 4,800	$ 35,617	$ 8,405
2015 and 2016 and 2018 Equity Inducement Plan \| Service Based Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vest period grant date				4 years
Share-based payment award, term				10 years
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares subject to options outstanding (in shares)				952
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)							3,023,650	104,561
Common stock available for future issuance (in shares)		7,228,113		7,228,113
Outstanding option awards (in shares)		3,247,677		3,247,677
2018 Equity Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for future issuance (in shares)		6,999,445		6,999,445
Outstanding option awards (in shares)		5,624,067		5,624,067
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)		1,000,000		1,000,000
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]
Pro-rated estimated fair value of options				$ 13,000
Stock-based compensation		$ 6,100	$ 2,700	13,000	$ 2,900
Unamortized expense		$ 4,400		$ 4,400
2016 Employee Stock Purchase Plan \| Employee Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued (in shares)		14,053		16,383
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)		14,053	704	16,383	2,496