Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Concentration Risk, Percentage	0
Securities owned, at fair value	$ 660,602		$ 311,039
Other investments	130,584		99,483
Securities sold, not yet purchased, at fair value	209,089		130,954
US Government Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt	1,009	[1]	9	[1]
Debt securities, interest rate			5.95%
US Government Securities | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	0.00%
US Government Securities | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.95%
Convertible Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt	7,501	[2]	5,958	[2]
Convertible Bonds | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	4.00%		5.00%
Convertible Bonds | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	10.00%		10.00%
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	430,941		176,939
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	574		324
Restricted Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	11,993		0
Warrants and Rights
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	3,370		5,912
Mutual Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Equity	16,382		525
Corporate Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Debt	188,832	[3]	121,372	[3]
Corporate Bonds | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.38%		3.38%
Corporate Bonds | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	11.54%		11.75%
Enterprise Master
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	6,482		6,841
Derivative, Fair Value, Net	0		(21)
Enterprise Master | Currency Forwards
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	0		(21)
Enterprise Master | Bank debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	6		5
Enterprise Master | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	2,516		2,677
Enterprise Master | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	973		973
Enterprise Master | Private Equity
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	413		406
Enterprise Master | Restricted Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	124		124
Enterprise Master | Rights
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	2,321		2,528
Enterprise Master | Trade Claims
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	128		128
Enterprise Master | Asset-backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	1		0
Merger Master
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	111,407		48,545
Derivative, Fair Value, Net	404		(156)
Merger Master | Currency Forwards
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	41		(10)
Merger Master | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	88,975		33,901
Merger Master | Options
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	455		200
Derivative, Fair Value, Net	(137)		(54)
Merger Master | Corporate Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	21,977	[4]	14,444	[4]
Merger Master | Corporate Bonds | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.38%		7.00%
Merger Master | Corporate Bonds | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	9.75%		10.88%
Merger Master | Cross Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	$ 500		$ (92)

[1] As of September 30, 2014, maturities ranged from May 2015 to April 2016 and an interest rates ranged between 0% to 5.95%. As of December 31, 2013, the maturity was April 2016 with an interest rate of 5.95%.

[2] As of September 30, 2014, maturities ranged from October 2014 to November 2014 and interest rates ranged between 4.00% to 10.00%. As of December 31, 2013, maturities ranged from May 2014 to October 2014 and interest rates ranged between 5.00% to 10.00% .

[3] As of September 30, 2014, maturities ranged from February 2015 to February 2046 and interest rates ranged between 5.38% to 11.54%. As of December 31, 2013, maturities ranged from January 2014 to February 2046 and interest rates ranged between 3.38% to 11.75%.

[4] As of September 30, 2014, maturities ranged from February 2017 to October 2022 and interest rates ranged between 5.38% and 9.75%. As of December 31, 2013, maturities ranged from April 2016 to October 2020 and interest rates ranged between 7.00% and 10.88%.