Financial Instruments and Other Inventory Positions Owned and Financial Instruments and Other Inventory Positions Sold, but Not Yet Purchased - Unrealized Gains/(Losses) on Derivative Instruments (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	$ (3,477)	$ (6,629)	$ (18,252)	$ (5,639)
Interest rate derivative contract | Investment banking
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	(483)	(880)	(775)	(2,052)
Interest rate derivative contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	(2,917)	(9,363)	(17,655)	(7,619)
Credit default swap index contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	(77)	3,495	178	3,884
Futures and equity option derivative contracts | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	$ 0	$ 119	$ 0	$ 148