The Company and Basis of Presentation (Details) $ / shares in Units, $ in Thousands				3 Months Ended	6 Months Ended	12 Months Ended	136 Months Ended
	Apr. 16, 2026 USD ($) $ / shares shares	Oct. 15, 2025 USD ($) $ / shares shares	Jun. 22, 2023	Jun. 30, 2026 USD ($) shares	Jun. 30, 2026 USD ($) segment shares	Dec. 31, 2025 USD ($) shares	Jun. 30, 2026 USD ($)	Sep. 06, 2024 USD ($)
Company and Basis of Presentation [Line Items]
Number of operating segments | segment					1
Proceeds from raising capital							$ 2,100,000
Accumulated deficit				$ 1,232,840	$ 1,232,840	$ 1,127,635	1,232,840
Cash, cash equivalents, and marketable securities				$ 1,100,000	$ 1,100,000		1,100,000
At-The-Market Offering, 2024 Sales Agreement
Company and Basis of Presentation [Line Items]
Sale of stock, authorized amount								$ 200,000
At-The-Market Offering, 2024 Sales Agreement | Common Stock
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares) | shares				0	296,396	445,668
Sale of stock, proceeds received					$ 9,700	$ 14,800
At-The-Market Offering, 2026 Sales Agreement
Company and Basis of Presentation [Line Items]
Sale of stock, authorized amount				$ 500,000	500,000		500,000
Issuance of common stock in connection with at-the-market and follow-on offering program, net of financing costs (in shares) | shares				0
Sale of stock, remaining authorized amount				$ 500,000	$ 500,000		$ 500,000
October 2025 Underwritten Offering | Common Stock
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares) | shares		17,094,594
Sale of stock, proceeds received		$ 296,400
Sale of stock, price per share (in dollars per share) | $ / shares		$ 18.50
Placement agent and other offering costs		$ 19,900
Over-Allotment Option | Common Stock
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares) | shares	975,000	2,229,729
April 2026 Underwriting Agreement | Common Stock
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares) | shares	7,475,000
Sale of stock, proceeds received	$ 435,200
Sale of stock, price per share (in dollars per share) | $ / shares	$ 62.00
Placement agent and other offering costs	$ 28,200
Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Asset acquisition, cash payment, threshold period (in years)			1 year