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, 2021	Dec. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (41)	$ (7,176)
Commodity Contracts | Principal Transaction Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	812	(8,107)
Other Contracts | Principal Transaction Revenue | TBAs
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	157	31
Other Contracts | Principal Transaction Revenue | Purchase Commitment
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(987)
Other Contracts | Other | Foreign exchange forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(22)	72
Other Contracts | ARS purchase commitments | Principal Transaction Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (1)	$ 828