Cash Equivalents and Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	$ 197,533	$ 58,210
Cash equivalents, Gross unrealized Gains	0	2
Cash equivalents, Gross unrealized Losses	0	0
Cash equivalents, Fair Value	197,533	58,212
Marketable securities, Amortized Cost	154,599	182,681
Marketable securities, Gross Unrealized Gains	189	176
Marketable securities, Gross Unrealized Losses	(15)	(48)
Marketable securities, Fair Value	154,773	182,809
Total cash equivalents and marketable securities, Amortized Cost	352,132	240,891
Total cash equivalents and marketable securities, Gross Unrealized Gains	189	178
Total cash equivalents and marketable securities, Gross Unrealized Losses	(15)	(48)
Total cash equivalents and marketable securities, Fair Value	352,306	241,021
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	197,533	45,722
Cash equivalents, Gross unrealized Gains	0	0
Cash equivalents, Gross unrealized Losses	0	0
Cash equivalents, Fair Value	197,533	45,722
U S Treasury Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost		12,488
Cash equivalents, Gross unrealized Gains		2
Cash equivalents, Gross unrealized Losses		0
Cash equivalents, Fair Value		12,490
Marketable securities, Amortized Cost		2,496
Marketable securities, Gross Unrealized Gains		0
Marketable securities, Gross Unrealized Losses		0
Marketable securities, Fair Value		2,496
Government agency securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	3,499	11,260
Marketable securities, Gross Unrealized Gains	2	22
Marketable securities, Gross Unrealized Losses	0	0
Marketable securities, Fair Value	3,501	11,282
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	151,100	168,925
Marketable securities, Gross Unrealized Gains	187	154
Marketable securities, Gross Unrealized Losses	(15)	(48)
Marketable securities, Fair Value	$ 151,272	$ 169,031

Fulcrum Therapeutics, Inc. 10-K