Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Jun. 22, 2023	Feb. 28, 2018	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2016	Jan. 01, 2023	Jan. 01, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation			$ 13,835	$ 1,709
Two Thousand And Fifteen Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares subject to options outstanding (in shares)			3,029
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)			7,393,885
Outstanding option awards (in shares)			2,996,404
Two Thousand And Fifteen And Two Thousand And Sixteen Equity Incentive Plan And Two Thousand And Eighteen 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
2018 Equity Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for future issuance (in shares)			6,029,000
Outstanding option awards (in shares)			5,384,241
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			$ 21,900
Stock-based compensation			5,400	$ 0
Unamortized expense			$ 16,500
2016 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued (in shares)			2,330	1,793