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, 2024	Dec. 31, 2023
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	$ 3,074	$ 2,933
Commodity contracts | Principal transactions revenue | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	3,083	2,842
Other contracts | Principal transactions revenue | TBAs
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	1	26
Other contracts | Other revenue | Foreign exchange forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income (pre-tax) Gain (Loss)	$ (10)	$ 65