Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	807,266,000		744,914,000
Operating Entities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	807,266,000		744,914,000
Operating Entities [Member] | US Government Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	206,123,000	[1]	182,868,000	[1]
Operating Entities [Member] | US Government Securities [Member] | Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	0.25%		0.25%
Operating Entities [Member] | US Government Securities [Member] | Maximum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	1.00%		8.00%
Operating Entities [Member] | Corporate Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	191,840,000	[2]	231,864,000	[2]
Operating Entities [Member] | Corporate Bonds [Member] | Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	3.13%		3.13%
Operating Entities [Member] | Corporate Bonds [Member] | Maximum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	13.50%		13.50%
Operating Entities [Member] | Convertible Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	12,633,000	[3]	18,130,000	[3]
Debt securities, interest rate			2.75%
Operating Entities [Member] | Convertible Bonds [Member] | Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.25%
Operating Entities [Member] | Convertible Bonds [Member] | Maximum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.75%
Operating Entities [Member] | Common Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	353,840,000		250,380,000
Operating Entities [Member] | Options Held [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	36,520,000		55,699,000
Operating Entities [Member] | Derivative [Member] | Receivables from Brokers-Dealers and Clearing Organizations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral posted	6,600,000		8,100,000
Operating Entities [Member] | Warrants and Rights [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	3,494,000		2,759,000
Operating Entities [Member] | Credit Default Swaps, Futures and Equity Swap Contracts [Member] | Other Assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative Assets	200,000		800,000
Operating Entities [Member] | Futures and Equity Swap Contracts [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative liabilities	800,000		800,000
Operating Entities [Member] | Mutual Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	2,816,000		3,214,000
Consolidated Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities owned, at fair value	2,234,000		6,334,000
Consolidated Funds [Member] | Corporate Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	0	[4]	401,000	[4]
Debt securities, interest rate			0.58%
Consolidated Funds [Member] | Commercial Paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	632,000	[5]	3,927,000	[5]
Consolidated Funds [Member] | Government Sponsored Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	1,602,000	[6]	2,006,000	[6]
Consolidated Funds [Member] | Government Sponsored Securities [Member] | Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	0.32%		0.32%
Consolidated Funds [Member] | Government Sponsored Securities [Member] | Maximum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	1.74%		1.74%
Enterprise Master [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	(15,000)		53,000
Securities owned, at fair value	5,824,000		5,827,000
Enterprise Master [Member] | Common Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	2,147,000		2,173,000
Enterprise Master [Member] | Distressed Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	57,000		0
Enterprise Master [Member] | Preferred Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	984,000		1,027,000
Enterprise Master [Member] | Private Equity [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	303,000		276,000
Enterprise Master [Member] | Restricted Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	29,000		47,000
Enterprise Master [Member] | Rights [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	2,171,000		2,173,000
Enterprise Master [Member] | Warrant [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	5,000		3,000
Enterprise Master [Member] | Currency Forwards [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	(15,000)		53,000
Enterprise Master [Member] | Trade Claims [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity securities	128,000		128,000

[1] As of June 30, 2012, maturities ranged from July 2013 to November 2013 and interest rates ranged between 0.25% and 1%. As of December 31, 2011, maturities ranged from November 2013 to November 2021 and interest rates ranged between 0.25% and 8%.

[2] As of June 30, 2012, maturities ranged from September 2012 to February 2041 and interest rates ranged between 3.13% and 13.50%. As of December 31, 2011, maturities ranged from January 2012 to February 2041 and interest rates ranged between 3.13% and 13.50%.

[3] As of June 30, 2012, maturities ranged from October 2014 to June 2016 and interest rates ranged between 5.25% and 5.75%. As of December 31, 2011, the maturity was August 2027 with an interest rate of 2.75%.

[4] As of December 31, 2011, the maturity was April 2012 with an interest rate of 0.58%.

[5] As of June 30, 2012, commercial paper was purchased at a discount and matures on July 2, 2012. As of December 31, 2011, commercial paper was purchased at a discount and matures on January 3, 2012.

[6] As of June 30, 2012, maturities ranged from October 2012 to May 2014 and interest rates ranged between 0.32% and 1.74%. As of December 31, 2011, maturities ranged from October 2012 to October 2013 and interest rates ranged between 0.32% and 1.74%.