Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments	$ 84,930,000		$ 59,943,000
Securities owned, at fair value	624,127,000		744,914,000
Operating Entities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments	84,930,000		59,943,000
Securities owned, at fair value	624,127,000		744,914,000
Operating Entities | Other Income [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Realized and unrealized gains/(losses) related to derivatives trading activities	7,800,000		7,000,000		1,900,000
Operating Entities | US Government Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	137,478,000	[1]	182,868,000	[1]
Operating Entities | US Government Securities [Member] | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	0.25%		0.25%
Operating Entities | US Government Securities [Member] | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	5.95%		8.00%
Operating Entities | Corporate Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	193,078,000	[2]	231,864,000	[2]
Operating Entities | Corporate Bonds [Member] | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	3.09%		3.13%
Operating Entities | Corporate Bonds [Member] | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	12.50%		13.50%
Operating Entities | Convertible Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	6,202,000	[3]	18,130,000	[3]
Debt securities, interest rate	5.00%		2.75%
Operating Entities | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	259,292,000		250,130,000
Operating Entities | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	2,332,000		250,000
Operating Entities | Options Held [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	20,546,000		55,699,000
Operating Entities | Warrants and Rights
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	2,354,000		2,759,000
Operating Entities | Derivative contracts, at fair value | Receivables from Brokers-Dealers and Clearing Organizations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral posted	6,700,000		8,100,000
Operating Entities | Credit Default Swaps, Futures and Equity Swap Contracts [Member] | Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative Assets	200,000		800,000
Operating Entities | 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	1,000,000		800,000
Operating Entities | Mutual Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	2,845,000		3,214,000
Consolidated Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments	204,205,000		228,820,000
Securities owned, at fair value	3,525,000		6,334,000
Consolidated Funds | Corporate Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Financial Instruments, Owned, Corporate Debt, at Fair Value	0	[4]	401,000	[4]
Debt securities, interest rate			0.58%
Consolidated Funds | Commercial Paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	1,614,000	[5]	3,927,000	[5]
Consolidated Funds | Government Sponsored Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities	1,911,000	[6]	2,006,000	[6]
Consolidated Funds | Government Sponsored Securities [Member] | Minimum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	0.28%		0.32%
Consolidated Funds | Government Sponsored Securities [Member] | Maximum
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, interest rate	4.00%		1.74%
Enterprise Master [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	6,000		53,000
Securities owned, at fair value	5,923,000		5,827,000
Enterprise Master [Member] | Debt [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	79,000		0
Enterprise Master [Member] | Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	2,680,000		2,173,000
Enterprise Master [Member] | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	997,000		1,027,000
Enterprise Master [Member] | Private Equity [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	297,000		276,000
Enterprise Master [Member] | Restricted Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	26,000		47,000
Enterprise Master [Member] | Rights [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	1,714,000		2,173,000
Enterprise Master [Member] | Warrant
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	2,000		3,000
Enterprise Master [Member] | Currency Forwards [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, Fair Value, Net	6,000		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 December 31, 2012, maturities ranged from November 2013 to November 2022 and interest rates ranged between 0.25% and 5.95%. 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 December 31, 2012, maturities ranged from January 2013 to February 2041 and interest rates ranged between 3.09% and 12.50%. 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 December 31, 2012, maturities ranged from May 2014 to July 2014 with an interest rate of 5.00%. 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 December 31, 2012, commercial paper was purchased at a discount and matures on January 2, 2013. As of December 31, 2011, commercial paper was purchased at a discount and matured on January 3, 2012.

[6] As of December 31, 2012, maturities ranged from August 2013 to December 2014 and interest rates ranged between 0.28% and 4.00%. As of December 31, 2011, maturities ranged from October 2012 to October 2013 and interest rates ranged between 0.32% and 1.74%.