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, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (2,327)	$ (132)
Commodity Contracts | Principal Transaction Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(2,362)	592
Other Contracts | Principal Transaction Revenue | ERROR in label resolution.
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(53)	371
Other Contracts | Other | Foreign exchange forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	15	(7)
Other Contracts | Auction Rate Securities Purchase Commitment | Principal Transaction Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 73	$ (1,088)