Fair Value Measurements - Fair Value Amounts of Derivative Instruments and their Effect on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 321	$ (1,830)	$ (7,647)	$ (2,416)
Commodity Contracts | Principal Transaction Revenue | Future [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(252)	(2,090)	(8,346)	(2,666)
Other Contracts | Principal Transaction Revenue | TBAs
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(2)	(86)	(14)	(76)
Other Contracts | Principal Transaction Revenue | Auction Rate Securities Purchase Commitment
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	555	350	691	332
Other Contracts | Other Income [Member] | Foreign exchange forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ 20	$ (4)	$ 22	$ (6)