Fair Value of Financial Assets and Liabilities - Aggregate Fair Values Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 13,765	$ 15,693	$ 23,141
Reclassification to permanent equity upon exercise			(374)
Change in fair value	(12,778)	(1,928)	(7,074)
Ending balance	$ 987	13,765	15,693
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Change in fair value of warrant liability
Embedded Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 10	10	10
Reclassification to permanent equity upon exercise			0
Change in fair value	0	0	0
Ending balance		10	10
Class C Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	13,755	15,683	23,131
Reclassification to permanent equity upon exercise			(374)
Change in fair value	(12,778)	(1,928)	(7,074)
Ending balance	$ 977	$ 13,755	$ 15,683

X4 Pharmaceuticals, Inc 10-K