Fair value measurements - Fair Value Amounts of Derivative Instruments and their Effect on Statement of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	$ (172)	$ 3,074
Commodity contracts | Principal transactions revenue | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	(189)	3,083
Other contracts | Principal transactions revenue | TBAs
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	$ 17	1
Other contracts | Other revenue | Foreign exchange forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)		$ (10)