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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	$ 19,454	$ 8,008	$ 24,255	$ 8,138
Interest rate derivative contract | Investment banking
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	479	844	999	1,369
Interest rate derivative contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	11,877	4,798	12,556	4,266
Credit default swap index contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	8,038	1,668	12,645	2,922
Equity option derivative contract | Institutional brokerage
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives Instruments	$ 18	$ 698	$ 53	$ 419