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 [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	$ 10,236	$ (4,511)	$ (6,208)
Interest rate derivative contract | Investment banking
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	(2,274)	(2,790)	(1,529)
Interest rate derivative contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	534	(1,678)	(2,511)
Credit default swap index contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	12,228	(1,080)	(1,522)
Equity option derivative contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	$ (252)	$ 1,037	$ (646)