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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (1,830)	$ 512	$ (2,416)	$ 1,475
Commodity Contracts | Principal Transaction Revenue | Future [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(2,090)	322	(2,666)	1,351
Other Contracts | Principal Transaction Revenue | TBAs
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	(86)	162	(76)	196
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)	350	(39)	332	(214)
Other Contracts | Other Income [Member] | Other TBA Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)		72		147
Other Contracts | Other Income [Member] | Foreign exchange forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Recognized in Income on Derivatives (pre-tax) Gain (Loss)	$ (4)	$ (5)	$ (6)	$ (5)