Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative Liability, Number of Instruments Held	38,221		10,070,267
Securities owned, at fair value	$ 320,737,000		$ 624,127,000
Other investments	99,483,000		84,930,000
Derivative, Fair Value, Net	377,000		202,000
Derivative liabilities	1,101,000		973,000
Derivative, Number of Instruments Held	71,129		28,133,185
Consolidated Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value			3,525,000
Other investments	187,480,000		204,205,000
Consolidated Funds | US Treasury and Government [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt			1,911,000	[1]
Consolidated Funds | US Treasury and Government [Member] | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			0.28%
Consolidated Funds | US Treasury and Government [Member] | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			4.00%
Consolidated Funds | Commercial Paper
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt			1,614,000	[2]
Enterprise Master
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	6,841,000		5,923,000
Derivative, Fair Value, Net	(21,000)		6,000
Enterprise Master | Currency Forwards
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	(21,000)		6,000
Enterprise Master | Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	5,000		79,000
Enterprise Master | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	2,677,000		2,680,000
Enterprise Master | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	973,000		997,000
Enterprise Master | Private Equity
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	406,000		297,000
Enterprise Master | Restricted Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	124,000		26,000
Enterprise Master | Rights
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	2,528,000		1,714,000
Enterprise Master | Warrants and Rights
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	0		2,000
Enterprise Master | Trade Claims
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	128,000		128,000
Operating Entities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	320,737,000		624,127,000
Other investments	99,483,000		84,930,000
Securities sold, not yet purchased, at fair value	137,527,000		177,937,000
Operating Entities | Other Income
Schedule of Trading Securities and Other Trading Assets [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities	4,400,000		7,800,000		7,000,000
Operating Entities | Derivative contracts, at fair value | Receivables from Brokers-Dealers and Clearing Organizations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral posted	10,000,000		6,700,000
Operating Entities | US Government Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt	9,000	[3]	137,478,000	[3]
Debt securities, interest rate	5.95%
Operating Entities | US Government Securities | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			0.25%
Operating Entities | US Government Securities | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			5.95%
Operating Entities | Convertible Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt	5,958,000	[4]	6,202,000	[4]
Debt securities, interest rate			5.00%
Operating Entities | Convertible Bonds | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.00%
Operating Entities | Convertible Bonds | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	10.00%
Operating Entities | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	176,939,000		259,292,000
Operating Entities | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	324,000		2,332,000
Operating Entities | Corporate Debt Securities [Member] | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	3.375%
Operating Entities | Options
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	9,698,000		20,546,000
Operating Entities | Warrants and Rights
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	5,912,000		2,354,000
Operating Entities | Mutual Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	525,000		2,845,000
Operating Entities | Futures, Forwards and Equity Swap Contracts [Member] | Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative assets	400,000		200,000
Operating Entities | Corporate Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt	121,372,000	[5]	193,078,000	[5]
Operating Entities | Corporate Bonds | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			3.09%
Operating Entities | Corporate Bonds | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	11.75%		12.50%
Merger Master
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	48,545,000
Derivative, Fair Value, Net	(102,000)
Merger Master | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	33,901,000
Merger Master | Options
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	200,000
Merger Master | Equity swap
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	(10,000)
Merger Master | Cross rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	(92,000)
Merger Master | Corporate Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	14,444,000	[6]
Merger Master | Corporate Bonds | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			7.00%
Merger Master | Corporate Bonds | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate			10.88%
Futures, Forwards and Equity Swap Contracts [Member] | Operating Entities | Accounts Payable, Accrued Expenses and Other Liabilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative liabilities	1,100,000		1,000,000
Options | Operating Entities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities sold, not yet purchased, at fair value	$ 6,573,000		$ 9,076,000

[1] As of December 31, 2012, maturities ranged from August 2013 to December 2014 and interest rates ranged between 0.28% and 4.00%.

[2] As of December 31, 2012, commercial paper was purchased at a discount and matures on January 2, 2013.

[3] As of December 31, 2013, the maturity was April 2016 with an interest rate of 5.95%. As of December 31, 2012, maturities ranged from November 2013 to November 2022 and interest rates ranged between 0.25% and 5.95%.

[4] As of December 31, 2013, maturities ranged from May 2014 to October 2014 and interest rates ranged between 5.00% to 10.00%. As of December 31, 2012, maturities ranged from May 2014 to July 2014 with an interest rate of 5.00%.

[5] As of December 31, 2013, maturities ranged from January 2014 to February 2046 and interest rates ranged between 3.38% and 11.75%. As of December 31, 2012, maturities ranged from January 2013 to February 2041 and interest rates ranged between 3.09% and 12.50%.

[6] As of December 31, 2013, maturities ranged from April 2016 to October 2020 and interest rates ranged between 7.00% and 10.88%.